More info needed for sure....
I get this sometimes after I have restarted the server a few times..... Normal browser refresh and log in works again..
No idea what causes it... And no idea if my very few users have noticed
After a quick search with the search engine of my choice I've found this stackoverflow question with a similar issue. There doesn't seem to be a solution but moving your repo out of node_modules and either symlink (by hand; as the answer describes) or npm link it.