It seems unlikely that your avatar path is the actual issue. Please read into your constraint violation.
1062 Duplicate entry '1-Mijheer.png' for key 'unq_folder_id__file_name': ALTER TABLE exp_assets_files CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci
You appear to be using the Assets add-on. The point where you are stopped in the upgrade is an update script that is running an
ALTER on all tables in your database, which fails when it reaches that Assets table:
ALTER TABLE exp_assets_files CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci
It fails due to the constraint. You have an integrity issue to sort out in that table clearly. That assets table has a
UNIQUE index based on the
file_name columns. You most likely have two (or more) rows in that table that have matching
file_name (which creates the index value,
You probably just have to query that table possibly like this:
SELECT * FROM
WHERE folder_id = 1
AND file_name LIKE "%Mijheer.png%";
And remove duplicate rows at your own discretion.
Do not do this without a secure backup of your database, and understand that removing rows from that table may result in incomplete records you'll have to repair after your upgrade
You might even be able to just run an Assets re-index before running the upgrade and that might fix it. Just try that anyways before your next upgrade attempt. Worth a shot.