This is now solved by disabling all scheduled tasks (ContentVersionCleanupPolicy, ScheduledRecordDeletion, and custom tasks). After that I deleted the TEMP folder and restarted the site. Cleaned the umbracoLog table, and deleted large json log files via Kudu. Now I could finally empty the recycle bin without getting timeout errors or lock errors. After this the site was running really smooth again, so I enabled the scheduled tasks again. 😃