Answers.Body: <p>Same happened to me. On installing new test CiviCRM under WordPress. The &quot;CiviCRM Contribution Page Widget&quot; plugin caused same error. Civi 5.27.1, WP 5.4.2,  php 7.2.24 on ubuntu 20.04 LTS</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-07-26T20:39:35.173Z
Answers.Text: Is this an Answer or just a 'me too'
Answers.UserDisplayName: petednz - fuzion
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-07-24T08:41:26.660Z
Answers.LastActivityDate: 2020-07-24T08:41:26.660Z
Answers.UserDisplayName: r4zoli
Answers.Body: <p>I encountered similar error messages attempting to activate CiviCRM 5.28.0 (but not on Plesk).</p>
  <p>The error in that situation was caused by a previous, failed installation attempt on the site this one had been cloned from.  Removing all CiviCRM files and tables then enabled the plugin to be activated correctly.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2020-08-17T19:20:35.773Z
Answers.LastActivityDate: 2020-08-17T19:20:35.773Z
Answers.UserDisplayName: Aidan
Body: <p>I’m trying to install a new copy of CiviCRM for Wordpress on Plesk Onyx. I get the following error when I try to activate CiviCRM after uploading the files to my server:</p>
  
  <p>Warning: require_once(CRM/Core/ClassLoader.php): failed to open stream: No such file or directory in /var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/civicrm.php on line 877</p>
  
  <p>Fatal error: require_once(): Failed opening required 'CRM/Core/ClassLoader.php' (include_path='.:/opt/plesk/php/7.2/share/pear') in /var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/civicrm.php on line 877</p>
  
  <p>I don’t know why this error is happening or how to fix it. I’ve never come across something like this before. As a result, I can't even install CiviCRM on my server.</p>
  
  <p>I'm running PHP 7.2.22, Wordpress 5.2.3, and CiviCRM 5.17.3.</p>
  
  <p>Thanks for your help</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-12T05:56:02.167Z
Comments.Text: Just to make sure, is there a file at `/var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/CRM/Core/ClassLoader.php`? Maybe something didn't get fully uploaded. Or maybe the file owner/permissions are wrong?
Comments.UserDisplayName: Demerit
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-12T19:12:23.010Z
Comments.Text: The file exists. I set the permissions to allow everything on that file. It's still not working.
Comments.UserDisplayName: com1tech
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-12T19:54:51.437Z
Comments.Text: @com1tech I suspect the permissions are the problem. Try making Classloader.php permissions 644.
Comments.UserDisplayName: Tommy B
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-12T20:22:28.997Z
Comments.Text: Classloader.php is set to 644. It still showing the same error.
Comments.UserDisplayName: com1tech
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-12T23:01:11.310Z
Comments.Text: Thanks everyone for your suggestions. I found that there was a plugin conflict that was causing the issue. After disabling the plugin, I was able to install CiviCRM.
Comments.UserDisplayName: com1tech
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-09-17T11:17:50.087Z
Comments.Text: Which plugin was it, out of curiosity? If it's a popular one than it may help others in the future to know :-)
Comments.UserDisplayName: Jade
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2020-01-22T06:10:30.003Z
Comments.Text: I encountered this also, it was the plugin "CiviCRM integration for Caldera Forms" which I'd installed before CiviCRM and this caused the problem described above.
Comments.UserDisplayName: Andrew Myers
Tags: <wordpress><error><installation><civicrm.settings.php><plugin>
Title: Error on CiviCRM Activation. Can't Install CiviCRM
ThreadText: # Error on CiviCRM Activation. Can't Install CiviCRM
  
  (Asked by: username_0 on 2019-09-12)
  
  I’m trying to install a new copy of CiviCRM for Wordpress on Plesk Onyx. I get the following error when I try to activate CiviCRM after uploading the files to my server:
  
  Warning: require\_once(CRM/Core/ClassLoader.php): failed to open stream: No such file or directory in /var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/civicrm.php on line 877
  
  Fatal error: require\_once(): Failed opening required 'CRM/Core/ClassLoader.php' (include\_path='.:/opt/plesk/php/7.2/share/pear') in /var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/civicrm.php on line 877
  
  I don’t know why this error is happening or how to fix it. I’ve never come across something like this before. As a result, I can't even install CiviCRM on my server.
  
  I'm running PHP 7.2.22, Wordpress 5.2.3, and CiviCRM 5.17.3.
  
  Thanks for your help
  
  **Tags:** wordpress, error, installation, civicrm.settings.php, plugin
  
  > **Comments:**
  > **username_1**: Just to make sure, is there a file at `/var/www/vhosts/ruinsideca.org/infoportal.ruinsideca.org/wp-content/plugins/civicrm/CRM/Core/ClassLoader.php`? Maybe something didn't get fully uploaded. Or maybe the file owner/permissions are wrong?  
  > **username_0**: The file exists. I set the permissions to allow everything on that file. It's still not working.  
  > **username_2**: @username_0 I suspect the permissions are the problem. Try making Classloader.php permissions 644.  
  > **username_0**: Classloader.php is set to 644. It still showing the same error.  
  > **username_0**: Thanks everyone for your suggestions. I found that there was a plugin conflict that was causing the issue. After disabling the plugin, I was able to install CiviCRM.  
  > **username_3**: Which plugin was it, out of curiosity? If it's a popular one than it may help others in the future to know :-)  
  > **username_4**: I encountered this also, it was the plugin "CiviCRM integration for Caldera Forms" which I'd installed before CiviCRM and this caused the problem described above.  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_5 on 2020-07-24. Score: 2
  
  Same happened to me. On installing new test CiviCRM under WordPress. The "CiviCRM Contribution Page Widget" plugin caused same error. Civi 5.27.1, WP 5.4.2, php 7.2.24 on ubuntu 20.04 LTS
  
  > **Comments:**
  > **username_6**: Is this an Answer or just a 'me too'  
  
  ---
  
  ### Answer by username_7 on 2020-08-17. Score: 0
  
  I encountered similar error messages attempting to activate CiviCRM 5.28.0 (but not on Plesk).
  
  The error in that situation was caused by a previous, failed installation attempt on the site this one had been cloned from. Removing all CiviCRM files and tables then enabled the plugin to be activated correctly.
  
  
  ---
  
  