Error in RSS feed

How to fix the error in the RSS link? Previously, everything worked, now when I open it, I see this error. Feed link. https://aviros.ru/search/sFeed,rss

XML parsing error: XML or text declaration not at the beginning of entity
Address: https://aviros.ru/search/sFeed,rss
Line 2, character 1:
<?xml version="1.0" encoding="UTF-8"?>
^
 

some issue with data most probably.
Check via source code: view-source:https://aviros.ru/search/sFeed,rss

But I don't see any problems there. I even restored the RSSFeed.php file from a backup, but it didn't help at all. 

Run validation and review xml source.
You have white space at very beginning, it probably does not like that.

Yes, you're right: a space.
In which file can I remove this space?
Could you please tell me the path to it?

Could be plugins theme or core. You must find it out

And how to find out?

Don't forget about me.And how to find out?   

You will need to stop asking questions and start working.
- disable all plguins and check
- change theme to default and check
- analyze core files, exit them step by step until you find culprit.

I've double-checked everything: the theme and plugins are not related, and only the core remains. I have no idea what to do. Should I reinstall the engine?

You may try to replace core files.

And....What are the main files?

Now I've done everything! I even reinstalled Osclass v8.3.1. I checked the theme and all the plugins. But nothing worked ((

We could not reproduce issue, means if you would run clean osclass installation you would not see this issue.
Hence, not abke to help you as its problem with plugin, theme, modification or something else printing extra white space.
Review also theme search.php and make sure at start no extra whitespace is added.

Can you tell me the path to the search.php file?