I hesitate to reset the access key on Computer A, my DLNA network and jremote work perfectly and have for years.
Just to be sure, did you reboot B after resetting its Access Key? Also reboot A (per Update below, I found it necessary to completely close/restart my MC server in order for it to allow connections from the client after resetting the client's Access Key)?
At least twice I ran into what may be a similar problem after locally restoring a client from a server library backup with "Restore Settings" checked, and then trying to have the client load/play the server library over the network. Each time I ended up resetting the server Access Key. It was an annoyance for sure, but everything worked after I entered the server's new Access Key into multiple MC PC clients and JRemote2 on Android phone. The server also runs BubbleUPnP to render on two Google Chromecast Audio dongles over DLNA - no problem there. So I don't think you have to be fearful of trying that server key reset (famous last words
, but you can backup the server before hand).
Update 8/5/2021:Today I find that if I (1) Update the client from a server library backup with Restore Settings checked, (2) Reset the client Access Key, (3) Close / restart the MC server (which was not done in description above), (4) Add the server library to the client using its Play from > Add Library... . (5) Then the server accepts a connection from the updated client using the original server Access Key in the Add Library... dialog box (after MC username, password authentication).
So if I "sacrifice" the old client Access Key (which I never use anyway), I don't have to reset the server access key when I have "Restore Settings" checked. Thereby I avoid updating that server key on the rest of the clients. Subsequent connections to the added server library do not require me to type in anything, even though I have Tools > Options... > Media Network > Authentication (username, password) checked. This should be analogous to the OP's case.
Again, if "Restore Settings" is unchecked when restoring a client from a server library backup, there is no need to reset any access keys, or re-add the server library if it was present before the restore. This is a consequence of left panel items and client access key not getting updated.
Sorry for long-winded babbling response, but it has taken me numerous iterations to understand all the ramifications of that single checkbox. I hope it helps someone.