Update from 9 to 10.0.3 ok but no more login

Hi,
I am having a dolibarr installation on a server (php+MariaDb I think) and I have upgraded from 9 to 10.0.3 without issues. I have followed the procedure and everything went fine until the end. I then put back the install.lock file, and I went to log into dolibarr.
I can reach the login page, but as soon as I insert the username and password, and I log in, I get a very simple message:
“… is currently unable to handle your request. HTTP ERROR 500”
I have checked all the permissions of the files, and they are ok.
I have then tried to downgrade to 10.0.2 and things went exactly the same way. I have then re-updated to 10.0.3.
Basically, I can upgrade/downgrade without issues, that section works fine. I can reach the login page, that section is fine, but I cannot go any further.
Can anybody give me some tips on how to troubleshoot, as I have no idea of what is causing the issue? Anything like a log file, or maybe even some more direct tips, are appreciated. Apart from following the procedure and uploading via FTP all the new versions files, I have not touched anything else. If anyone think it could be any additional third party software that might conflict with the new update, is there a way to disable those programs without having to intervene too drastically onto dolibarr? It’s not a production platform, so I could reinstall it from scratch, but in the past months I have spent time configuring it and customizing it with some extra free dolibarr software, having to re-start all over would honestly put me off.
Thanks in advance

Hello,

Error 500 means php error
please check the logs to know why

I cannot find the dolibarr logs… or should I check the PHP logs?

You have to check the PHP logs

Here is some of the stuff I have found:
22010 fx2.it [Tue Nov 26 00:08:37 2019] [error] [client 188.29.3.66:0] AH01276: Cannot serve directory /home/sites/fx2.it/public_html/crm/: No matching DirectoryIndex (index.cgi,index.pl,index.asp,index.html,index.html.var,index.shtml,index.php,index.php3,index.htm,default.htm,default.asp) found, and server-generated directory index forbidden by Options directive
22069 fx2.it [Tue Nov 26 00:09:08 2019] [error] [client 188.29.3.66:0] AH01276: Cannot serve directory /home/sites/fx2.it/public_html/crm/: No matching DirectoryIndex (index.cgi,index.pl,index.asp,index.html,index.html.var,index.shtml,index.php,index.php3,index.htm,default.htm,default.asp) found, and server-generated directory index forbidden by Options directive
22275 [Tue Nov 26 00:10:06 2019] [error] [client 188.29.3.66:0] PHP Fatal error: Cannot declare class DolibarrDebugBar, because the name is already in use in /home/sites/fx2.it/public_html/crm/htdocs/custom/debugbar/class/DebugBar.php on line 0

Hi,

I understand now, you have DolibarrDebugBar module.
It should be removed, because DolibarrDebugBar is now in the CORE.

Please delete /home/sites/fx2.it/public_html/crm/htdocs/custom/debugbar

Thank you so much! As soon as I have deleted the file you indicated, I managed to log in immediately!

Greatly appreciated!