One more to check off: once you have the machine running, go to the 'Options' menu in Advanced view, and choose 'Other options...'. In the dialog that opens. is the middle checkbox - 'Run the client?' - checked? It needs to be.

But please check - using task manager or similar - whether the CLIENT (boinc.exe) is running or not. That makes a huge difference to the answer to your question.

What you can try is exit BOINC completely, then navigate to the BOINC data directory, remove gui_rpc_auth.cfg and restart BOINC (Manager).
That will put a new empty gui_rpc_auth.cfg file into the data directory.

Another thing you can try is to actually add a password into the file. Can be anything.
Just don't end with a carriage return (Enter). Just fill in a password on the first line and save the file.

Then in BOINC Manager do File->Select computer, fill in localhost and the password, click OK.
If all's well, this should be done only once.

One last thing you can check is whether there have been any changes to the (Windows) firewall. It may be necessary to set inbound and outbound rules for boinc.exe and boincmgr.exe (and where asked, use tcp port 31416)

Were there any clues in the Event log you recall ? Had you shutdown Boinc or rebooted for example ?

I ask because I'm bound to 331.76 dev driver for a couple of weeks, so can't really look at 331.82 yet.

Copy the BOINC program and data folders, then install BOINC on top of them and you should get everything back.