I am also having this issue, I cannot figure out exactly what triggers it… but the only way for me to advance is closing Rhino + Grasshopper and restarting them.
I am running:
Rhino Version 6 SR29
(6.29.20238.11501, 08/25/2020)
Grasshopper Version 1.0.0007
on Windows 10 Enterprise
Version 1909, OS build 18363.1016