Hello,
Yes, that's very likely the cause. A query running for over 6000 seconds and locking the hikashop_user table will block any operation that needs to access that table, including order processing and status changes.
Here's what I'd recommend:
1. Kill the stuck query : ask your hosting provider to kill that specific process, or if you have phpMyAdmin access, go to the "Processes" tab and kill it from there.
2. Check AcyMailing's synchronization settings : this query is coming from AcyMailing trying to sync email addresses with HikaShop. There might be a misconfigured or overly broad sync task that's causing it to run on too many records at once. Look in AcyMailing's configuration for the HikaShop user synchronization options and make sure they are properly configured.
3. Once the query is killed, your orders should start processing normally again and you should be able to change order statuses.
If the problem recurs, it would be worth checking with the AcyMailing support team about why their sync query takes so long.