Ok, i found now some Error Entries, if someone try to reopen the Session.
[Tue Aug 20 06:28:40 2019] [-:error] [client XXXXXXXXXXXXXXXX:47421] Debugging: Did you remember to make the first column something unique in your call to get_records? Duplicate
value '2' found in column 'category'. in \n* line 1254 of /lib/dml/mysqli_native_moodle_database.php:
call to debugging()\n* line 1400 of /lib/dml/moodle_database.php: call to mysqli_native_moodle_database->get_records_sql()\n* line 218 of /calendar/externallib.php: call to moodle_database->get_records_select()\n* line 1444 of /webservice/lib.php:
call to core_calendar_external::get_calendar_events()\n* line 1290 of /webservice/lib.php: call to webservice_base_server->execute()\n* line 44 of /webservice/rest/server.php: call to webservice_base_server->run()\n
[Tue Aug 20 06:28:42 2019] [-:error] [client XXXXXXXXXXXXXXXXX:47421] Default exception handler: Notwendiger
Parameter "username" fehlt Debug: \nError code: missingparam\n* line 494 of /lib/setuplib.php: moodle_exception thrown\n* line 566 of /lib/moodlelib.php: call to print_error()\n* line 34 of /login/token.php: call to required_param()\n