Website not visible return status “No input file specified” server Debian 10/ispconfig 3.2/apache2.4

I have a configuration multi site with virtual hosts in a dedicated server (o.s Debian 10, ispconfig 3.2, apache 2.4). I upgraded few days ago from Debian 7 to Debian 10. I have at least 20 sites that are in a folder "/home/www". Only one of these sites is not visible. I made all the possible modifications also considering the new syntax apache "require all granted". I tried really everything but I always get the error "no input file specified". This site has an index.php (it includes an other php inside a subfolder). If i changed the content of index.php, nothing changes. Then the problem isn't the content, also because another site with the same structure is perfectly working. The only way to see the site is to change the extension in htm but I need php. I suppose that the problem could be "php" but I can't see the errors (oddly the error.log of the site is empty). The php version used is 5.6 but I have also the most recent version 7.3 of php installed on my server. I dont know any more what to do. I hope in someone helping me. Thanks

 

It is the same. I can't see any difference. I have always had the sites outside var/www and I preferred to keep them in this folder. In any case the other sites are working. If you want I can share the vhost conf files of this site. I restored to the old version after a useless try with the new syntax of apache 2.4 (require all granted). Thanks for your support

 

I'm not so skillful with ispconfig. I'm learning. The great problem is that I can't see any log file. Oddly the error log file is empty (not for other sites).

 

I changed php version for the site (also if I'm afraid that an old php script could not work any more) and at last I saw the log error file. No more the error "no input file specified". Now I have this error (also by replacing with a simple php)
€‹[Thu May 27 22:44:45.024419 2021] [fcgid:warn] [pid 20775] (104)Connection reset by peer: [client 93.35.219.230:24345] mod_fcgid: error reading data from FastCGI server
[Thu May 27 22:44:45.024442 2021] [core:error] [pid 20775] [client 93.35.219.230:24345] End of script output before headers: index.php

 

Does anyone help me? I can't understand the reason of this error. Maybe is there a way to investigate about the problem by tracing the php activity?