Surely is something related to Subscriptions translation files:

I did delete them from server and resetting tabs via toolbox is working.
I put them back and the error comes back again.

Actually there are many single quotes like this: ‘

But I’m sure they are present in many many many italian translations. we use them a lot… why the error comes up just with subscriptions ones?

Anyway I’ll remove all single quotes from sp-subs-it_IT.po and then I’ll feedback again.

Fixed: just removing single quotes from 

wp-contentsp-resourcesforum-languagesp-pluginssp-subs-it_IT.po

and everything works nice… actually now I see some translation in subscriptions strings that were not translated before.

still valid question: Why this happens only with subscriptions plugin?

ps: all this edits fixed all problems on a mirror website. Unfortunately the issue related to my older post 

https://simple-press.com/support-forum/sp5-general-topics/problem-with-post-edit-buttonow-users-can-subscribe-the-entire-forum/

is still open

good question. we are trying to get to the bottom of it. Another user re-ran PO-EDIT on the .po file from glotpress which indicates its not our translation data and the wp glotpress tool itself…

but we are still researching…