Thanks for all your help @rjen
I've fixed the problem - it was an old theme that the DL part of TP was looking for but the path to the theme in SMF was incorrect.
It is nice that you found the problem: I am a bit confused though, because I cannot reproduce the error in any way:
1. Tried to delete the previously assigned theme: TinyPortal falls back to the default...
2. Tried by deliberately breaking the theme paths: TinyPortal page shows up without formatting, but it does show...
Would you mind sharing how
exactly you managed to break it so it would result in the
Unable to load the 'main_above' template error? Perhaps we can prevent such errors in future...