Ummmmm.... SUCCESS!
Stick your clock back a year. Can't believe I fail to try it before. Not the most elegant of solutions but gets me out of a hole for now.....
Will let you know on the toolbox...
At last! someone else is going through the same problem!
I noticed this happen about last week on two K550's (Identical CISS)
Have tried everything as you have, got one working with a different CISS used for my K5400 (which us currently in pieces).
However the new CISS has a pigment ink...