HTTP Internal server error

Your computers list shows a large number of machines. It's very difficult to give you a meaningful response if you don't at least identify the machine having the problem.  In any case the recent "server issues" are not likely to be relevant to you since you have lots of decent and high end GPUs that wouldn't be crunching the BRP4 tasks that were affected.  Was the machine actually crunching BRP4?

The "no user name" in the account column is a much different problem.  It's telling you that the machine is not part of your main account.  Why don't you (in BOINC Manager) select that project line and click the 'Properties' button to see what shows as 'User name', 'Team name', 'Computer ID', 'Host location', and anything else that might be of interest.  At least with the 'Computer ID' entry, others would be able to see what is going on with that particular machine.

At the moment, it seems that the computer is no longer part of your account so wouldn't even be one of the ones currently listed in your full list.  However, the ID it used to have before you started your OS re-install should be easy for you to spot.  If you look through your full list and identify that ID, it might be possible to bring it back 'into the fold', so to speak.

Will you please stop flailing around - you're probably just making things worse!

In reference to the above:-

1. The name you use is not relevant - nobody else but you can see it.
2. The host ID of 12864006 is all that is needed.  The link to previous tasks is not needed.
3. I don't have the faintest idea of what "c&p" is supposed to mean.  Anyway, don't do it!
4. MeerKAT 0.13 means you run BRP7 tasks on your RX 6900 XT GPU - it's not the BRP4 server issue.

If the boinc client is running, please stop it.  Identify the directory where BOINC is installed and see if you have two files there - account_einstein.phys.uwm.edu.xml and client_state.xml.  If those exist, I want you to do just two things and report back.

1. In the account file there is an <authenticator> string - 32 random-looking characters.  Check if this string is exactly the same as the same string from the same file on one of your working machines.  Check with a plain text editor and don't make any changes.
2. In the state file, find the following three parameters, <host_venue>, <rpc_seqno> and <hostid>.  They are all in the Einstein <project> section of the file.  What are the current values of those parameters?  Once again, don't make changes, just report.

EDIT:  When checking the authenticator for agreement with the one on a working host, don't post the value (of either) in a reply.  Just report if they agree or not.