I am experiencing something very similar. Like you, it is a DD6900 and we are on CV v11.20. We also have experience an issue when the AuxCopy is run to tape. We  do get chunks that say it is unreadable, whilst performing the AuxCopy, and then subsequently knocks over boostFS. We have had the Dell Engineering team engaged on this for  quite a while now, and the  challenge that I face is that the  failure pattern does seem to change , and which is really difficult to pin down.

We initially had a Media Agent on Server 2012R2, and that was a completely deleted and reloaded as a Win 2016 STD server. We do still get failures, and in our case the Media Agent gets hung up. The media agent is an old HP DL380 Gen 8 , which probably is not helping our situation. Once we experience an issue it’s a reboot of the media agent that fixes the situation temporarily. Upon looking at the most recent issue, I noticed that the “use unbuffered I/O” was not ticked. In most situations you would use unbuffered I/O but I am not sure about unbuffered with DD. Anyone got any suggestions?

what are the used BoostFS version and the DD DDOS versions? On the DD one can run below command, to show information from the connected clients using the boost protocol:

ddboost show connections

And how the boostFS is used and setup? Using the security option lockbox and not the intergration with AD using Kerberos? To get an idea? Compressed restore (if high enough version is used) still set to the default "false"?

As backups are working, I assume DD settings like having distributed-segment-processing is enabled https://www.dell.com/support/manuals/en-uk/data-domain-boost-file-system/dd_p_boostfs_7.11_win_configuration_guide/prepare-the-system-for-boostfs?guid=guid-e0e0796b-8f6b-4d99-a0ea-d58698f4f2f4&lang=en-us ?

https://elabnavigator.dell.com/vault/pdf/DD_BoostFS_CommVault_All.pdf?key=1582731303123 "Data Domain BoostFS CommVault All Simple Support Matrix June 19, 2023" states for CV Simpana v11 that win2019 would support :
Data Domain Operating System:
DD OS 7.10.x, DD OS 7.11.x, DD
OS 7.3.x, DD OS 7.4.x, DD OS
7.5.x, DD OS 7.6.x, DD OS 7.7.x,
DD OS 7.8.x, DD OS 7.9.x

and BoostFS:
7.10, 7.11, 7.3,
7.4, 7.5, 7.6, 7.7,
7.8, 7.9

I can't really see that much to configure things for boostFS 7.11 during install https://www.dell.com/support/manuals/en-uk/data-domain-boost-file-system/dd_p_boostfs_7.11_win_configuration_guide/dd-boostfs-for-windows-configuration-guide-7.11?guid=guid-5b8de7b7-879f-45a4-88e0-732155904029&lang=en-us?