Try to measure voltage in different points. First put 5V in jacks and but DCV and 100V range. Then measure points
S6/A3 - S4/B1 = 5V
S2/B3 - A4/1 = 5V
A4/6 - A4/1 = 5V
A4/5 - A4/1 = 0,5V
A4/3 - A4/1 = 0,05V (From your first post with fault it read about 0,0689V)
If there is something funky business (there should be) try different range and see those results.
You should also try wave the switches and see if that affects reading. That way you might see if switch is faulty or something.
I was also thinking that fault might be in switch S11 between A6-A5. If there is a leakage it will affect just 100V range but it should drop voltage not raise it.