Hi @nicleics,
This must be a pretty frustrating situation and you absolutely have our sympathies.
What I’d suggest at this point is installing WordPress in a subdirectory per my earlier suggestion and testing things out there – that way, we could gain some useful insights and see if everything is fine within your hosting environment with just a plain copy of WordPress, The Events Calendar and Events Calendar PRO before other factors are introduced.
I realize you stated you weren’t comfortable with that process earlier in this thread: is that still your feeling? If so is there anyone who could help you with this? Alternatively, our offer of a refund certainly still stands should it be easiest to try out a different solution.