Since today (as far as I know) one site does not load and returns a 500 error. When debugging it shows the error:
"Attempted to load class "Helper" from namespace "HelixUltimate\Framework\Platform".
The odd thing is:
- I have 8 sites on the same server created from the same base
- None of them have this issue except one
- When switching template it's gone (but the whole site is)
- Checked PHP, extensions, and so on, will continue tomorrow
- Other topics haven't been of help, posting here 
Details are in 'hidden.' Will update if I find something myself.
--> fixed: Reverted back to back-up, nothing there, just cannot upgrade latest Joomla now, will look into this later <--