5N2 Won’t Do Time Machine Backup

I have 2 iMacs and a 5N2 on an ethernet network. I set up iMac #1 for Time Machine backups months ago, and it has performed without a problem. However, iMac #2 will not perform a TM backup. TM says it is waiting to complete the initial backup, but nothing is happening. Why would one iMac backup work fine, and the second will not perform? I also get these messages 'The backup disk could not be found. Make sure the backup disk is connected or select a different backup disk" "Time Machine couldn’t complete the backup to “Drobo5N2.local”. Any help would be appreciated.

I’m not a Mac User, so take this guesswork for what it’s worth, but check
Can the target be accessed using normal means (Finder?)
Does TimeMachine allow backing up two machines to the same target?
If so, are separate subdirectories required to do so?

The second two of these would be potential problems, but possible to work around, using Windows built-in backup.

Unless it’s something super simple like that I’m not going to be able to assist beyond these very basic suggestions of things to check & you’ll need to wait for a Mac user.

Thanks for your reply. Yes, finder sees the Drobo and all of the shares on it. However, it seems Time Machine can’t find it. I have installed separate Time Machine shares for iMac #1 and iMac #2. The first has been working fine for months, but not so for #2.