I was able to sign my extension a couple of days ago for the first time using the jpm sign command. The last 2 days however after I made some updates and increased my version number in the package.json, the sign command is returning:
JPM [error] FAIL
Error: Validation took too long to complete; ā¦
I even tried rolling back the code and resigning but no joy 
Any help would be appreciatedā¦ tHX
Same problem as you. I think it is related to problems with Mozilla servers. There is another thread in the forum reporting the same problems by other users.
https://discourse.mozilla-community.org/t/cannot-upload-new-add-on-version-server-error/
I wonder if this was also possible linked to the number of warnings I was gettingā¦ After I submitted this for a manual review it seemed to work fine from thereā¦ Never found the root cause though