Perhaps it is because it's trial version, but that would mean that the trial version is not working only on Opera. I've installed PM and the extensions for Chrome and Mozilla and it works just fine, Opera is the only one having problems with it, ESET can't install the extension. Maybe it has something to do with the browser itself.
Thank you for your help anyway! I'll see whether it works when I buy the full license, if not, I'll open the support ticket.