My problem has been solved (with much help from a Presonus technician). I just loaded the previous version 9244 of the firmware on the RM32AI and the system didn't freeze anymore, the CS18AI kept working and the recording via the by firewire connected MacBook with Capture2 went OK. I've got a problem with strange behaviour of channels due to an backupped scene witch had been changed while the latest version of the firmware was installed on the RM32AI and recalled under the 9544 firmware version. I deleted the specific scene and build it from scratch based on the "Zero out board" scene. All problems are gone ;-)
My current installed firmware & software versions
StudioLive CS18AI Firmware v13840
StudioLive RM32AI Firmware v9544
Capture 2 v2.4.0.49805
Universal Control 2.9.2
Apple MacOS version Mojave 10.14.1