Hello,
finally after many nights and hours of dispair, I think i found a work around to an calibration issue that occured with my Celestron AVX and my new iOpton CEM70 and asiair pro:
issue:

after west-east RA movement the following backlash correction failed after trying many steps, with the final error star did not move enough (attention this is always a DEC backlash correction before the DEC calibration not RA as many maybe expected!)
the steps counted up very slowly before the error message came
there was no visible movment of the star, so error message was right
repeating the calibration process several times, selecting new stars, switching on/off sometime it brought success sometimes not (with AVX higher success rate then with CEM70), sometimes i was not able to get one successsful calibration the whole night)

interesting is, when using the handcontrolers or the asiair pro slew arrows north/south (DEC) worked without any visible backlash problem. I check visual with eye piece and there was no backlash visible bigger than 3 sec (AVX) or 1 sec Cem70. Check gear and belt tension mount (CEM70) was perfect.

This indicates that it is not a mount problem, as usally most discussed and at first sight expected, it is an asiari issue!!! when trying to make the backlash correction before starting the DEC calibration, asiair seems to give no slew pulse to the mount!

solution/workaround:
by accident i found following workaround that works in my case always(!).

whenever backlash correction is slow and there are more than 5 steps, stop calibration and guiding
with running asiair, plug out USB cable to mount from asiair (dont switch asiair off!)
wait 2-3 sec and plug in usb cable again
goto mount menu in asiair software and disconnect and reconnect mount (necessary or not, i do)
(make new image and plate solve with cam or guiding cam as main cam)
(make new sync after plate solve)
start calibration process again
after this unplugging and reconnecting, in my case calibration succesds to 100%, backlash correction is very fast no star did not move enough issue., maximum 3 steps and 3sec.
I think there is a bug in the mount communication of asiair only during backlash correction (DEC) during calibration. Zwo please check your code! Maybe there is a new initialisation of the communication after unplugging and reconnect the usb cable. (and no, it is not the cable, tried several ones and AVX and CEM70 have different cables and connectors!)

ohh yeah, and once before PA using 0.001s exposure time helps to get rid of the exposure time error, as discribed in another post.

I hope this workaround helps not only in my case!

Markus