H Slawomir,
AtoM 2.5 used MySQL 5.7, while in version 2.6 we have moved to MySQL 8.0, where this new collation format is supported. Also, not sure if you are trying to upgrade in place, but our Upgrade instructions typically recommend installing a new version of AtoM (following the new installation documentation for that version), and then loading in the data to this new instance. My first question is: is your new 2.6 instance using MySQL8? See:
If yes:
Did you remember to drop and recreate the database first? See steps 3 and 4 from the Upgrading notes
here:
3. If you’re upgrading from 2.5.x or lower to 2.6.x or higher make sure your data is on utf8mb3 or utf8 (the default if you followed the installation documentation) and that you are using MySQL 8.0 as that’s a requirement since AtoM 2.6. During the upgrade task, your data will be transformed to the utf8mb4 charset and the utf8mb4_0900_ai_ci collation.
4. Re-create the database with the new charset and collation:
mysql -u username -p -e "DROP DATABASE IF EXISTS atom;"
mysql -u username -p -e "CREATE DATABASE atom CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci;"
This is typically done before you load your 2.5.4 data, so the collation format should be correct before you run the upgrade task. Please let us know if this helps!
Cheers,