Hi HEN,
Based on my understanding, you have a SharePoint Online Access database, you recently encountered "Access Services is unable to process the request” error.
Based on my experience, as you have used this database for nearly one year, the error occurred can be caused the performance problem. To test this, I suggest you create a new database using Access, and publish it to SharePoint Online, to test if you can reproduce the issue.
In addition, I suggest you check the Storage of your SharePoint Online tenant:
In your team site, click Site Actions, click Site Settings, click Go to top level site settings under Site Collection Administration; click Storage Metrics under Site Collection Administration to check the storage of your SharePoint Online tenant.
We suggest you to re-upload the database for the testing, as you are not able to do this in your situation, it’s okay not to do this testing.
Any updates, please feel free to post.
Thanks,
Evan Zhang