Hello Rudolf,
For find where error in joomla , you may do
1. Turn on Joomla Max Error report and Development mode - in Global Joomla settings - so you will see full error path and error details
2. You may try In Joomla Manage - Disable All 3 party Joomla Extensions and switch to Default Joomla Template - and next - turn on extensions one by one
But this better do on website copy - with that you not kill something imports
3 But best way ask extensions developers - Yes, first you may Backup your website - for not lost something important
In
OrdaSoft Joomla Gallery - best way for not lost photos - do Gallery update without uninstall first version
Regards
OrdaSoft team