when the Advanced Modul Manager Pro form nonumber.nl is activated.
I got from "nonumber Peter van Westen" this answer
That is an issue with the module. It is using custom fields in the wrong way.You'll have to report the issue to Joomla51.
We must Upgrade our WEBSites to Joomla 3.4.5. With Joomla Version 2.5 there is no Problem.
NoNumber Advanced Modul Manager 5.3.3 PRO
NoNumber Framework 15.11.2151
NoNumber Extension Manager 5.2.1
Please Help
Markus Lehmann