Ah yes, sorry, I missed that somehow. I can confirm that using AuthMechs PLAIN fixes the problem.
Unfortunately not -- I tried both with Homebrew (using brew install --HEAD isync) and building directly from git. Same error in both cases.
A few more data points: $ mbsync -v isync 1.3.1 The same config works on another Ubuntu machine I use (that one uses isync 1.3.0). When I add the verbose -V flag, I see the following on macOS: Authenticating with SASL mechanism PLAIN-CLIENTTOKEN... Error: SASL(-7): invalid parameter supplied: Parameter Error in /BuildRoot/Library/Caches/com.apple.xbs/Sources/passwordserver_saslplugins/passwordserver_saslplugins-192.30.1/plain_clienttoken.c near line 195 but on Ubuntu I see Authenticating with SASL...
SASL error when connecting to Gmail on macOS