Topic on Project:Support desk

Resolved by myself )

Hello, I have following wiki website:

MediaWiki: 1.29.1
PHP:5.6.30 (cgi-fcgi)
MySQL: 5.6.35-81.0

So I have downloaded Flow, add extension to my LocalSettings.php file, run the update.php. And when I go to discussion page flow appears but when I add a topic it cannot be posted to discussion page, instead of it I see see this (when I click post message):

Could someone tell me how can I fix it? Thank you

Is something related shown in the Console and/or Network tab of your browser's developer tools? Have you checked the log of your webserver if there are some errors? Also see https://www.mediawiki.org/wiki/Manual:How_to_debug

So I have found out the error occurs during the posting, it says:

<br />
<b>Fatal error</b>:  Class 'ApiErrorFormatter_BackCompat' not found in <b>/var/www/u0402490/data/www/wikireshebnik.ru/includes/api/ApiMain.php</b> on line <b>283</b><br />

Any ideas how to resolve the problem?

The problem is resolved by reuploading core MW files to the server