We see quite sporadical errors on sending instant messages via:
Participants of a course --> dropdown: send a message

The error occuring on website in browser  looks like:
formatnotsupported
webservice/formatnotsupported

Error log:
PHP Notice:  Undefined offset: -1 in /message/externallib.php on line 252...
PHP Notice:  Trying to get property 'smallmessage' of non-object
PHP Notice:  Undefined offset: -1 in /message/externallib.php on line 253
PHP Notice:  Trying to get property 'fullmessageformat' of non-object in message/externallib.php on line 253

Unfortunately the error is occasionally and not very often.
The message text is plain text without special chars.