I am getting the same error but haven't updated the software past 1.5.5
This build has been working great for quite some time and suddenly I am unable to calibrate guiding and get the error.
GUIDE_RA error: 'mount or backlash problems
[2021-03-25T01:10:54.889 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 22 Direction: RA_INC_DIR Duration: 2000 ms.
[2021-03-25T01:10:54.889 EDT INFO ][ org.kde.kstars.ekos.guide] - "Calibration rejected. Star drift is too short. Check for mount, cable, or backlash problems."
GOTO functionality works fine after a plate-solve. Guiding from a saved backup works fine. Here is a portion of the log which shows the final error. Something is amiss in the mount itself as the same error occurs on a fresh SM 1.5.5 installation.
[2021-03-25T02:44:47.157 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration # 17 : STAR 943.36 , 627.996
[2021-03-25T02:44:47.158 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 17 Direction: RA_DEC_DIR Duration: 8000 ms.
[2021-03-25T02:44:47.159 EDT DEBG ][ org.kde.kstars.ekos.guide] - Star x pos is 7.02242 from original point.
[2021-03-25T02:44:55.335 EDT DEBG ][ org.kde.kstars.ekos.guide] - Capturing frame...
[2021-03-25T02:44:55.422 EDT DEBG ][ org.kde.kstars.ekos.align] - Mount motion finished.
[2021-03-25T02:44:57.039 EDT DEBG ][ org.kde.kstars.ekos.guide] - Received guide frame.
[2021-03-25T02:44:57.039 EDT DEBG ][ org.kde.kstars.ekos.guide] - Tracking Square QRect(927,611 32x32)
[2021-03-25T02:44:57.054 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration # 18 : STAR 943.085 , 627.331
[2021-03-25T02:44:57.054 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 18 Direction: RA_DEC_DIR Duration: 8000 ms.
[2021-03-25T02:44:57.055 EDT DEBG ][ org.kde.kstars.ekos.guide] - Star x pos is 7.69011 from original point.
[2021-03-25T02:45:05.331 EDT DEBG ][ org.kde.kstars.ekos.guide] - Capturing frame...
[2021-03-25T02:45:05.366 EDT DEBG ][ org.kde.kstars.ekos.align] - Mount motion finished.
[2021-03-25T02:45:07.025 EDT DEBG ][ org.kde.kstars.ekos.guide] - Received guide frame.
[2021-03-25T02:45:07.025 EDT DEBG ][ org.kde.kstars.ekos.guide] - Tracking Square QRect(927,611 32x32)
[2021-03-25T02:45:07.066 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration # 19 : STAR 943.121 , 626.827
[2021-03-25T02:45:07.066 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 19 Direction: RA_DEC_DIR Duration: 8000 ms.
[2021-03-25T02:45:07.067 EDT DEBG ][ org.kde.kstars.ekos.guide] - Star x pos is 8.19367 from original point.
[2021-03-25T02:45:15.302 EDT DEBG ][ org.kde.kstars.ekos.align] - Mount motion finished.
[2021-03-25T02:45:15.332 EDT DEBG ][ org.kde.kstars.ekos.guide] - Capturing frame...
[2021-03-25T02:45:17.213 EDT DEBG ][ org.kde.kstars.ekos.guide] - Received guide frame.
[2021-03-25T02:45:17.215 EDT DEBG ][ org.kde.kstars.ekos.guide] - Tracking Square QRect(927,610 32x32)
[2021-03-25T02:45:17.288 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration # 20 : STAR 942.703 , 626.345
[2021-03-25T02:45:17.288 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 20 Direction: RA_DEC_DIR Duration: 8000 ms.
[2021-03-25T02:45:17.289 EDT DEBG ][ org.kde.kstars.ekos.guide] - Star x pos is 8.68095 from original point.
[2021-03-25T02:45:25.332 EDT DEBG ][ org.kde.kstars.ekos.guide] - Capturing frame...
[2021-03-25T02:45:25.550 EDT DEBG ][ org.kde.kstars.ekos.align] - Mount motion finished.
[2021-03-25T02:45:27.002 EDT DEBG ][ org.kde.kstars.ekos.guide] - Received guide frame.
[2021-03-25T02:45:27.002 EDT DEBG ][ org.kde.kstars.ekos.guide] - Tracking Square QRect(926,610 32x32)
[2021-03-25T02:45:27.007 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration # 21 : STAR 942.991 , 626.306
[2021-03-25T02:45:27.007 EDT DEBG ][ org.kde.kstars.ekos.guide] - Iteration 21 Direction: RA_DEC_DIR Duration: 8000 ms.
[2021-03-25T02:45:27.008 EDT DEBG ][ org.kde.kstars.ekos.guide] - Star x pos is 8.7168 from original point.
[2021-03-25T02:45:27.015 EDT INFO ][ org.kde.kstars.ekos.guide] - "GUIDE_RA: Scope cannot reach the start point after 21 iterations. Possible mount or backlash problems..."
I am not sure where to look next and my problem isn't from updating to a more recent build.