Answers.Body: <p>This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by Brad Bell in his comment on the original question.</p>
  
  <p>Upgrading to Craft 3.3.16.1 fixes this issue.</p>
  
  <p>(I'm posting this as an answer in case anybody else runs into the same issue.)</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2019-12-01T20:42:23.080Z
Answers.Text: The upgrade fixed the issue for me, too
Answers.UserDisplayName: aaandre
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2019-11-23T10:48:56.703Z
Answers.LastActivityDate: 2019-11-23T10:48:56.703Z
Answers.UserDisplayName: Mark Northrop
Body: <p>Craft 3.3.15</p>
  
  <p>Previous version of the entry was saved 1 minute ago, so no changes in settings between the two entries.</p>
  
  <p>Any ideas on how to debug this?</p>
  
  <p>Update - here's the error, which looks very much like what @mtnorthrop is getting:</p>
  
  <pre><code>2019-11-20 10:41:44 [-][1][-][error][TypeError] TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/helpers/ElementHelper.php:47
  </code></pre>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-11-15T05:14:28.330Z
Comments.Text: Check your Craft `storage/logs` files for `[error]` - that should contain the underlying error message for the "unknown error" flash message.
Comments.UserDisplayName: Brad Bell
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-11-21T10:25:02.163Z
Comments.Text: I've also experienced this issue. The logs report: TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /vendor/craftcms/cms/src/helpers/ElementHelper.php:47
Comments.UserDisplayName: Mark Northrop
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-11-21T22:18:19.530Z
Comments.Text: @mtnorthrop did you find a solution?
Comments.UserDisplayName: aaandre
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-11-23T00:26:39.157Z
Comments.Text: This was a bug in Craft that's been fixed as of Craft 3.3.16: https://github.com/craftcms/cms/blob/develop/CHANGELOG-v3.md#3316---2019-11-22 Going to go ahead and vote to close this as a bug report.
Comments.UserDisplayName: Brad Bell
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-11-23T10:42:15.437Z
Comments.Text: @aaandre I can confirm that updating to Craft 3.3.16.1 fixed this issue for me.
Comments.UserDisplayName: Mark Northrop
Tags: <error><bug><entry-versions><preview>
Title: If I try to preview a previous revision of an entry, I get "an unknown error occurred"
ThreadText: # If I try to preview a previous revision of an entry, I get "an unknown error occurred"
  
  (Asked by: username_0 on 2019-11-13)
  
  Craft 3.3.15
  
  Previous version of the entry was saved 1 minute ago, so no changes in settings between the two entries.
  
  Any ideas on how to debug this?
  
  Update - here's the error, which looks very much like what @mtnorthrop is getting:
  
  ```
  2019-11-20 10:41:44 [-][1][-][error][TypeError] TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /Users/andrei/Desktop/dev/hl/mailbuilder/vendor/craftcms/cms/src/helpers/ElementHelper.php:47
  
  ```
  
  **Tags:** error, bug, entry-versions, preview
  
  > **Comments:**
  > **username_1**: Check your Craft `storage/logs` files for `[error]` - that should contain the underlying error message for the "unknown error" flash message.  
  > **username_2**: I've also experienced this issue. The logs report: TypeError: Argument 1 passed to craft\helpers\ElementHelper::isTempSlug() must be of the type string, null given, called in /vendor/craftcms/cms/src/controllers/EntryRevisionsController.php on line 473 and defined in /vendor/craftcms/cms/src/helpers/ElementHelper.php:47  
  > **username_0**: @mtnorthrop did you find a solution?  
  > **username_1**: This was a bug in Craft that's been fixed as of Craft 3.3.16: https://github.com/craftcms/cms/blob/develop/CHANGELOG-v3.md#3316---2019-11-22 Going to go ahead and vote to close this as a bug report.  
  > **username_2**: @username_0 I can confirm that updating to Craft 3.3.16.1 fixed this issue for me.  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_2 on 2019-11-23. Score: 2 | Accepted answer
  
  This bug was due to an issue with the Stringify library and was fixed in Craft 3.3.16.1, as stated by username_1 username_1 in his comment on the original question.
  
  Upgrading to Craft 3.3.16.1 fixes this issue.
  
  (I'm posting this as an answer in case anybody else runs into the same issue.)
  
  > **Comments:**
  > **username_0**: The upgrade fixed the issue for me, too  
  
  ---
  
  