Found it, the GTN750 GPS plugin was the problem.
I use that only for GA planes so tested this as the last plugin not thinking it would gave problems because I do not use it in the 737. It turns out it is not the plugin itself, but If you open it from the XP plugin menu, it makes a .ini file in the directory of the plane you are flying. It also promotes itself to master-gps. And even if you do not ever use it , as long as the .ini file is there, it stays master. I have not tried it for the 737 but you can disable the master status in the GTN menu and still use it as a slave, I do that in some GA planes. So maybe an option if you want to use in the 737 as a tablet.
By the way, now librain is not working, the rain effect of the 737 is not bad but I had never noticed it before because it seems to be only active if I zoom in a little or look from an angle. Not an ideal position during flight (but moving closer does not activate it). It could use some higher drop density (maybe add some layers over each other ) but it looks pretty realistic. It could be a nice base for a librain replacement. The only thing I really miss in Vulcan.