[otrs] Some follow-ups are not received

We have encountered the following problem. Some follow-ups do not reflect in OTRS - the system neither attach them to a correspondent ticket nor creates a new one. Nevertheless OTRS sends New ticket auto response with the wrong ticket number (like 2008120410000901, while our SystemID is 20). Of course no ticket in the system with this number. System Log has no info on the above action. OTRS ver.2.3.3. Is there any additional info I should provide? Boris