Andy I started getting this same error back in March after an upgrade on my mailbox, and some other internal re-arrangement of my other VM's. Other users on the same Grommunio VM were not affected. Whilst I don't normally use the Desktop app I do use the WebApp and this is where I got the error every time I logged in.
Initially I also found clicking on OK cleared the dialogue box with no further issue. However, after a week or so I noticed I could not open Files from the WebApp (I normally use Files via the Mac Nextcloud client), even if I cleared and re-entered the account details. Files was working fine using the Nextcloud clients on Mac OS, IOS and Android so just waited for an update to fix it.
When the new 15-04 ISO was released I spun up a new VM and migrated my data using the process both @Andy and myself have publish elsewhere in this forum on a number of occasions. On testing the new VM I had exactly the same issue, so I started thinking this must be caused by corruption of some kind in one of the databases I had copied from the old VM to the new 15-04 ISO VM.
I spun up another 15-04 ISO VM but this time instead of migrating my mailbox over I created new accounts in the latest VM and migrated the data by using Outlook to export my mailboxes to PST. I then imported the PST's into the lastest VM and the error was gone and Files now works properly again within the WebAPP.
So sometime back in March I must have picked up some corruption possibly in the Grommunio database and this was causing the popup and the inability to use Files when logging into Grommunio via the WebApp.
Published this in case other forum users come across similar issues, to this, then you may have to rebuild your mailboxes without direct migration.