guests encounter with blank page

[corsobg]

Guests can open topic pages but not any page with index.php

I.e. - no problem opening hxxp:domain.com/forum/an-interesting-topic [nonactive] but a blank screen on hxxp:domain.com/forum/index.php [nonactive]

SMF 2.0.17
Mods:    Pretty URLs 2.2; Simple Audio Video Embedder 5.0.4;Gaming nation theme
SMF cache cleared and caching disabled altogether
repair_settings run
PHP version 7.0.25
No errors in log
Site running on add-on domain, shared hosting 

Any ideas?

[Illori]

did you check the server error log? an error has to be logged somewhere.

[corsobg]

Nothing there

[corsobg]

Just to add some info:

If I disallow guests to browse the forum, the URL hxxp:domain.com/forum/index.php [nonactive] opens the warning that the forum can be accessed by registered users only and the log in window.

P.S. While experimenting I found out that the issue seems to be related to the language pack used (Bulgarian UTF8) If the default language is set to English, the forum will be visible.