Office js Outlook API returns invalid JWT token for EWS - jwt

Question

I have an add-in in Outlook that has the ReadWriteMailbox permissions, but when I call getCallbackTokenAsync({ isRest: false }, callback), the token I get back has no signature. Basically, the token I'm getting is in the format xxxxxx.yyyyyy (instead of xxxxxx.yyyyyy.ZZZZZZ).
This appears to have just started happening in the last 2 days, however, the last time I worked on this project was about 4 months ago so it may have started happening sometime before now, but I'm just now seeing the error.
The EWS server is Office 365 (https://outlook.office365.com/EWS/Exchange.asmx).
I have also noticed that calling getCallbackTokenAsync has returned the same token, minus the signature the last 2 days.

Answer 1

After almost pulling my hair out with this, I got it to work.
The original problem of getting an invalid JWT was the culmination of several problems (or suspected problems). The JWT was actually valid, although expired, but when I dumped the value out to the F12 tools console while debugging, the value was truncated to 1024 characters. I discovered that here: https://stackoverflow.com/a/27844847/4520915. The JWT was expired because it was using a token from the cache instead of requesting a new one from EWS. I fixed that by by closing Outlook, clearing IE's cache, and re-opening Outlook as suggested by the Outlook Add-ins Team - MSFT.
The issue now was getting EWS to accept the token because I kept getting a 401 error ("Access is denied. Check your credentials and try again."). I discovered this was because I was running the add-in and the subsequent Azure Function that it utilizes on localhost. Apparently, EWS doesn't like localhost. After deploying the add-in to a development environment, we all good.
Thanks to the Outlook Add-ins Team - MSFT for their help.