Problem with translation showing random strings

Hello,

I am having strange problem with the translation of a theme. Sometimes __ or _e returns random strings, part of translation or empty strings. The translation is in Cyrillic.
What’s even stranger, is that on the same page it can appear/disappear after refresh, completely random.

The problem appeared after I moved my site from our dev server to the client’s server.

Anyone has an idea what’s going on?

Thanks in advance