@peopleinside thanks for your help! The dev mode was already enabled, and I was basically digging through the logs trying to understand the issue.
Anyways, it seems to be the htaccess issue, not something PHP-wise.

Accidentally, I've got everything working by applying part of the default WordPress htaccess file

Maybe this will help somebody. Here is my public/.htaccess file:

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteRule ^index\.php$ - [L]
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule . /index.php [L]
    RewriteCond %{HTTP:Authorization} ^(.*)
    RewriteRule .* - [e=HTTP_AUTHORIZATION:%1]
</IfModule>

Thanks a lot! I think this can be closed as Solved

Hi @peopleinside ! Thanks a lot for your quick replies.
I've tried the thread you've mentioned, but it didn't help. I've tried to apply this suggestion several ways:

Option #1

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{REQUEST_FILENAME} !-f
    #RewriteRule ^(.*)$ ./index.php?/$1 [QSA,L]
    RewriteCond %{THE_REQUEST} ^GET.*index\.php [NC]
    RewriteRule (.*?)index\.php/*(.*) /$1$2 [R=301,NE,L]
    RewriteCond %{HTTP:Authorization} ^(.*)
    RewriteRule .* - [e=HTTP_AUTHORIZATION:%1]
</IfModule>

This resulted in 404 errors for any page I am trying to access.

Option #2

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^(.*)$ ./index.php?/$1 [QSA,L]
    RewriteCond %{THE_REQUEST} ^GET.*index\.php [NC]
    RewriteRule (.*?)index\.php/*(.*) /$1$2 [R=301,NE,L]
    RewriteCond %{HTTP:Authorization} ^(.*)
    RewriteRule .* - [e=HTTP_AUTHORIZATION:%1]
</IfModule>

This didn't make any difference from the original problem that I've faced with XHR acting weird. Everything works except XHRs, e.g. at the Agents page.

I'll try to give more insights into our setup below:

• I believe this is UVdesk 1.0.17
• PHP 7.4.27
• I've installed UVdesk via SSH, using composer and this guide https://www.uvdesk.com/en/blog/open-source-helpdesk-installation-on-ubuntu-uvdesk/
• I have my virtual host pointing right into /public folder
• The htaccess was already in place after installation, I didn't add it manually
• The first error "No input file specified." I got right away, when accessing the website root. It is shown for any URL, if I don't manually add "index.php" to the URL
• I've just added a question mark to this line:

RewriteRule ^(.*)$ ./index.php?/$1 [QSA,L]

As per some suggestions on Symphony forums.

Any suggestion where to dig?

Thanks in advance!

After the installation, I was getting a “No input file specified” error. To fix it, I’ve changed this

RewriteRule ^(.*)$ ./index.php/$1 [QSA,L]

To this

RewriteRule ^(.*)$ ./index.php?/$1 [QSA,L]

in .htaccess as was suggested on the internet.

Which helped, but I’ve got another problem.

It resulted in 500-error for Ajax on this page
/en/member/agents

Somehow, the XHR request URL looks like this: http://<mydomain>/en/member/agents/xhr?%2Fen%2Fmember%2Fagents%2Fxhr=

NOTE: Everything works fine if I add “index.php” in the URL itself. E.g.:
http://<mydomain>/index.php/en/member/agents
Then the XHR request looks like:
http://<mydomain>/index.php/en/member/agents/xhr

I feel like I'm missing some simple stuff, but can't Google anything for over 4 hrs already... Any help? Thanks!