Hello,
  thanks for answering. I am on the following:
OS			
			
									Linux Debian 9
PHP						
									7.3.16 
MySQLi						
									8.0.18 
Joomla 3.9.18
Actually, I installed SEBLOD as first and unique extension. When i enable Multisite from Global Config > Seblod 3.x > Multi-site tab, just clicking on Yes button and then Save & close i get:
An error has occurred.
						1064 You have an error in
 your SQL syntax; check the manual that corresponds to your MySQL server
 version for the right syntax to use near 'groups, public_viewlevel, 
viewlevels, configuration, options FROM #__cck_core_s' at line 1								
			
Also, any webpage on joomla server report 404 error and what i can do is to revert to last snapshot. I actually did not investigate more.