The +02:00 is the UTC offset… your time is 2 hours later than UTC, so subtract 2 from your local time to get UTC.
This could absolutely be causing averageSince to return null. Since the log entry shows the correct offset, it looks like your database is using system time and the clock is set to the wrong time. I would just go check that, but if you want to dig deeper, on the server running MariaDB, run…
mysql -u root -p
show GLOBAL VARIABLES like 'time_zone';
If time_zone comes up as SYSTEM, your system clock is incorrect. If it comes up as something other than your timezone or SYSTEM, you’ll need to find out how/where it got changed and correct it.