InvalidXMLResponse

You ask for input, I share input :)
error: The server returned something vdirsyncer doesn't understand. Error message: InvalidXMLResponse('Invalid XML encountered: mismatched tag: line 21652, column 2574\nDouble-check the URLs in your config.')
error: While this is most likely a serverside problem, the vdirsyncer devs are generally interested in such bugs.

Error only occured once this morning.

from syslog:
Feb 12 06:39:09 server davmail-server[2750]: request: <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages"><soap:Header><t:RequestServerVersion Version="Exchange2013_SP1"/></soap:Header><soap:Body><m:GetItem><m:ItemShape><t:BaseShape>IdOnly</t:BaseShape><t:AdditionalProperties><t:FieldURI FieldURI="calendar:RequiredAttendees"/><t:FieldURI FieldURI="calendar:ModifiedOccurrences"/><t:FieldURI FieldURI="calendar:OptionalAttendees"/><t:ExtendedFieldURI PropertyTag="0x3008" PropertyType="SystemTime"/></t:AdditionalProperties></m:ItemShape><m:ItemIds><t:ItemId Id="AAMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgFRAAgI26BHOzyAAEYAAAAA8AOVsiSIb06SaOtFnI7+ngcAvyZVHSUhZU6JlRPgJI7hBAAAAAABDQAAvyZVHSUhZU6JlRPgJI7hBAADfGR/KgAAEA==" ChangeKey="DwAAABYAAAC/JlUdJSFlTomVE+AkjuEEAAN8eXkG"/></m:ItemIds></m:GetItem></soap:Body></soap:Envelope> Feb 12 06:39:09 server davmail-server[2750]: 2025-02-12 06:39:09,184 WARN [CaldavConnection-36080] davmail - Item AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_KgAAAA==.EML not available: /users/user@example.com/calendar/AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_KgAAAA==.EML Feb 12 06:39:09 server davmail-server[2750]: 2025-02-12 06:39:09,309 WARN [CaldavConnection-36080] davmail - Item AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR-_gAAAA==.EML not available: /users/user@example.com/calendar/AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR-_gAAAA==.EML Feb 12 06:39:09 server davmail-server[2750]: 2025-02-12 06:39:09,476 WARN [CaldavConnection-36080] davmail - Item AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_jgAAAA==.EML not available: /users/user@example.com/calendar/AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_jgAAAA==.EML Feb 12 06:39:09 server davmail-server[2750]: 2025-02-12 06:39:09,639 WARN [CaldavConnection-36080] davmail - Item AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_cwAAAA==.EML not available: /users/user@example.com/calendar/AQMkADgyODBmZTQ2LTg1NmMtNGFkOS1iZmI4LTMxZWMwZTg3YTA4MgBGAAAD8AOVsiSIb06SaOtFnI7-ngcAvyZVHSUhZU6JlRPgJI7hBAAAAgENAAAAvyZVHSUhZU6JlRPgJI7hBAADfGR_cwAAAA==.EML

Start with the InvalidXMLResponse report and the logged DavMail SOAP response, focusing on the mismatched XML at line 21652, column 2574. Compare the server response with vdirsyncer's handling of invalid XML and determine whether the failure can be reproduced. Done means the cause is identified and a verified fix or clear server-side diagnosis is documented.