an error occures im mirror of the motor unit

[prepressure]

My heidelberg herkules linotype imagesetter showns an error: "an error occured in the mirror of the motor unit"
sometimes is shown this error sometime not.
Any idea

[frailer]

Intermittent errors are the worst kind. Places you in service-tech territory. If you are still able to output film, and it looks "OK", then you have the option of pushing on and doing nothing...until it gets worse/stops working. Probably in that order.

[prepresstex]

Check the vacuum system it is in one of hte long sides you need to clean the filters and let all the water come out the mirror works on air bearing