Carriage stops „shock front“ while nothing visible ist stuck/broken

Dear Kniterate Community,

Unfortunately, we have a problem with our machine. It knits normally, but it gets stuck at two  points with the error message "Shock Front". All needles look good, and we've already blown out the machine to detect any small parts. Without luck. After that, we slowly pushed the carriage over the needles to see if there's anything visible in the carriage itself. Also without success. Now we're out of ideas.  We also did the jack check successfully. They look good too. 
We looked under and inside the carriage but everything looks normal too.

Maybe you have a tip? We would be very grateful, as we urgently need the machine for a graduate collection!!

Thanks for reading!