To expand on what deerough said:

It seems that sometimes the running processes of BOINC as well as BOINC itself is the root of this issue. So a possible solution before rebooting as well is closing Gridcoin, killing BOINC and all associated processes (including processes spawned by the projects - i.e. vina, wcg, etc), and then restarting BOINC and Gridcoin.