EWS NTLM often whines that password is not correct

I've successfully connected to a hosted Exchange server through EWS and NTLM authentication (kudos BTW). But there is a problem - often during operations against EWS I'm presented with "Login to account XXX failed" "Login to server YYY with account XXX failed". I can ignore it (cancel) and it will work for seconds or minutes but eventually reappear. Sometimes it's very frequent, sometimes is here and there. TL;DR; connection works, I can read directories and e-mails. From looking at development tools I see these scenarios: 1. POST requests return 200 always (no messsage of course) 2. Eventually a POST would return 401. Then Thunderbird issues a GET, it returns 401 as well but response includes header ''WWW-Authenticate: NTLM XXXXXX''. Then another GET is issued containing header "Authorization" with the value from previous response. This time it gets 200 and continues without POST requests without a dialog. 3. Sometimes there are more POST requests with 401 (I assume in parallel) before a GET is issued and then I see a dialog. Sometimes it recovers (regardless of whether I cancel or retry), sometimes gets stuck with 401 with dialog reappearing each time it tries something. This case it's similar in sense that thunderbird issues GET with 401 and WWW-Authenticate header, issues another GET with Authenticate header but still gets 401. If I had to guess I'd say the problem happens when POST requests are issued in parallel, not sequentially. OTOH it might be a problem at the server, but it isn't very likely.