I had the same exact problem after migrating a r710 to a r420 dell server. I used the same lto7 HH& ibm tape drive in both server yet it wouldn't catalog, inventory any older tape. In my old server I was using a lsi sas HBA 6gbs adapter and in the new server i used the second HBA port on my PERC H830 adapter. I could write to blank tape of format old tape but couldn't restore any old data !!!!
I finally put another LSI sas HBA adapter that i had over from another server in my R420 and now everything works fine, i can read my older tape.
The issue was that my lsi sas adapter gave me multiple choice for block size and was set up at 1024k block size but my Perc H830 gave me only 3 choices and 512k was the biggest block size i could set on my tape unit from within veeam. With the new sas adapter I have all the options up to 1024k.
hope it helps someone as I spent about 8 hours trying to figure out what was wrong.