Adding to this topic my own experience that might help others who hit a dead end with the instructions above. I discovered the following by working with a Plastic Support member.
Problem: I was also getting the "Invalid credentials, username or password are not valid." error after changing my password, and the above instructions didn't fix the issue for me.
The Fix:
I was instructed to
1) Close Plastic app and associated game engine
2) Navigate to WIN: %localappdata%\plastic4\ or MAC: $HOME/.plastic4/ and to rename that folder and append _bak (backup to it)
3) Re-open Plastic app, and re-login (this got me in, but I didn't have my workspaces, so I...)
4) Close Plastic, and then copy over relevant files from plastic4_bak/ -> plastic/
plastic.workspaces (this will restore your workplace association when you re-open plastic next)
hidden_branches.conf (this will re-hide any branches you had hidden)
ignore.conf (this will restore any ignores you had set up)
This fixed the issue for me, and kept most of my settings intact. Hope this helps someone else!
