Apache has an access and an error logfile. You can also set the logging behaviour in your apache config. Increase the log level, you should be able to get more detailed information about your 503 error.
When your hard disk/partition is full, log files can’t be written (happened to me once )
I tried putting the highest log level on apache and still nothing. In access.log I have the error 503 which appears as many times as I try to refresh the page of the nextcloud. I checked but I have plenty of space available on the hard drive
So for now I’m still stuck with this damn error 503
On apache, the default setup uses mod_php. All other modules are a bit problematic and unfortunately we don’t have a default configuration you can test. There were several issues over the time but I don’t know if anybody managed to set up a fully working NC.
You can as well use nginx for a smaller footprint where default configs are available.
I FOUND THE SOLUTION!
Instead of put ProxyPassMatch ^/(…php(/.)?)$ fcgi://127.0.0.1:9000/var/www/nextcloud/$1 in my apache vhost, i need to put <FilesMatch ^(.+\.php)(/.+)$> SetHandler "proxy:fcgi://localhost:9000/" </FilesMatch>