I coded a python application which was running OK as a cron job. Later I added some libraries (e.g. pynotify and other *) because I wanted to be notified with the message describing what is happening, but it seems that cron can't run such an application.
Do you know some alternative how to run this application every five minutes? I'm using Xubuntu.

import gtk, pygtk, os, os.path, pynotify

I can run the application without cron without problems.
Cron seems to run the application but it won't show the notification message. In /var/log/cron.log there are no errors. The application executed every minute without problems.
my crontab:
*/1 * * * * /home/xralf/pythonsrc/app
thank you
I don't see any problem in cron job with pynotify? What is the error you are getting?
Can you run your python code separately to check whether your python code is working really well but only fails with cron?
Celery is distributed job queue & task manager written in Python but it may be too much for your needs.
Supervisord also can do some sort of cron task if you know that your program shall close in 5 minutes. So you can configure supervisord to start the task soon after. None of them are not easier like cron job.
Notification as a cron job