Drobo 5D3 - time machine stopped backing up

My last backup to my Drobo 5D3 was December 11th, before the last upgrade to Catalina. Now I get the notification on my Mac that Time Machine backup failed. It also says

“Files can’t be copied onto the backup disk because it is read-only. You may need to repair or erase the disk using Disk Utility. If the disk can’t be repaired, select a different disk for backups.”

Disc utility can’t repair the disk. The Drobo works fine otherwise (I have a lot of my photos there and Lightroom works with them fine). Is this a Drobo/Apple issue or is there something I can do to fix it.

All my software is up to date.