I'm surprised that you have problems with plate solving in KStars 3.6.6 on Ubuntu.
I follow this thread ~ since beginning but since 4 weeks I'm using a polar wedge and run AZ-GTi in EQ-Mode.
Plate Solving works now perfect and I'm completly excited with AZ-GTi in EQ-Mode.
But perhaps EQ-Mode is the reason why Plate Solvings works so good now.

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