I overhauled the starter and cleaned all ground and power connectors, won't start. Yesterday I got help from my wife and was able to check the voltage also when starting. Battery had 12,6 V, but when starting, it dropped under 7 V. Somehow my Ctek XS 3600 charger seems to destroy batteries during winter (battery was one year old Exide AGM battery, 19 Ah). It sure isn't any plug in and forget -charger. I got new battery yesterday evening, just before the local part shop's closing time. With new battery it works brilliant again.
Thinking positive, the starter should have been overhauled anyway quite soon and specially the ground connectors are quite common reasons for starting problems also, so it's good all are checked and clean now.
Bad thing is that we still got 5 cm new snow last night, next few days it will still be just around zero, so not yet riding.