When I reduced the length of the RG6, I also reduced the length of the SMA patch cable to 10cm. After this change, I had some QSOs, no complains. However, when I looked at my own spectrum, I was not so happy seeing splatter on the wrong side-band.
Probably, my converter, outputting 200mW, saturated the PA, which input is rated 100mW. Just as an experiment, I changed the patch back to 100cm. Having had some test transmissions, I can now state that my signal is clean again.
Sometimes, it only needs a little bit of coax in the right place to solve a problem.