I just tried a real life polar alignment with the plate solving method using your suggested 'workaround', by setting an arcsecond-per-pixel value, instead of the arcminute width and the PA, and plate solving performed without any problem. I had "Refresh solver failed" errors before but not this time. Furthermore, the errors get updated correctly contrary to before where the errors would not always get updated and often not displayed.
William Optics GTF81 on Avalon M-Zero OBS
ZWO ASI1600MM Pro
ZWO ASI290MM (guide)
ODroid N2 running Ubuntu Mate 20.04 LTS
KStars 3.6.0 Beta