I am using my own Virtual box installation for work (non BOINC related), running tasks on the host machine (Win10, 64b). Any time Virtual box is running a linux guest server, the manager stops finding the localhost as a place where the tasks can run and gives an error (mostly erroneous password).

Shutting down the virtual machine brings all back to normal, but is no option. What can I do to have manager work again?

You don't have to run BOINC Manager to run work on BOINC, it's only needed when you want to control the client. So what you can do is run the client (boinc.exe) from a command line (boinc.exe --detach, or boinc.exe --detach_console). You will have to start the client from the install directory, as it's not in Windows its path: "c:\program files\boinc.exe" --detach (with the quotes, as there's a space in the path)

But other than that it sounds as if your VirtualBox is using port 31416 which BOINC uses to communicate between its parts.
From an elevated command prompt, try running netstat -ab and wait until this runs out, this can take several minutes. If you want to run it to a text file, do netstat -ab > path.to/textfile.txt, make sure the path exists.

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