Sins some days, I am not able to add an Author or a Category to my blog posts.

The same error with a different url i thrown when I access the Dashboard of the Developer and Members Sections:

I tried to look at the files there but the umbraco/backoffice folder does not exists.

We used Umbraco Version 7.6.6 and the uSkinned minimal Corp Starter Kit / Theme. I also installed uWebSize, Config Tree, Copy Paste Grid Editors, RankOne - SEO Toolkit, Formulate and CM Umbraco Tools.

To solve the problem I already tried:

1. upgraded all Packages to the newest version
2. uninstalled the last installed Package (CM Umbraco Tools)
3. Upgraded Umbraco to Version 7.9.2

None of these tasks could resolve the problem.

Does anybody know how to solve this error or what else I can try to solve it?