Hi! I've just installed your JPrc Cronjobs extensions on Joomla 3.4.4, PHP Version: 5.5.9-1ubuntu4.9. When I create a new task, I can't change the „Published”, and „Enable log” parameters and the „Cronjob Expression” value stays empty despite of changing the Minutes, Hours, etc. The Permissions tab in the Category also empty. I reinstalled the extensions, but the situation didn't change.
What is the solution for this problems?
We have just tested XTCronjob on Joomla 3.4.4 and the very same PHP Version 5.5.9-1ubuntu4.11 / Database Version 5.5.44-0ubuntu0.14.04.1.
It has worked OK. We have created a category and executed a fopen task to retrieve a page.
In you case, if you can't even create a category, then it is a general problem (category management is based on Joomla core feature, actually not a feature that we code and it is integrated via configuration).