Hi, I can confirm this behaviour, which seems to be at least a little odd.
I can't find a way to delete the global "General" category from any of my projects (this might be a misconfiguration on my part, but I can't seem to find a way to configure a project to NOT use any of the global categories).
Even though I do have a couple of project-specific categories, I can not remove any of the global ones.
The second odd behaviour is, that I can not ever delete the "General" category from the global categories - even though there are a couple of other global categories defined.
If I try to ever remove that category, I get "APPLICATION ERROR #1504".
- create new project
- create new categories within that project
- try to remove the global "General" category
- create new Project
- create new global category
- try to delete the global "General" category
Maybe this does make sense in some way and I have just misconfigured my installation in some way - but then again, this is really really shady to get, how to properly configure a project so that you do NOT have to use that GENERAL category...