Now further into the bug and - promised - my last post on this. I find this to be an absolutely inacceptable bug. I found out that obviously the latest Cakewalk version for unknown reason is unable to handle certain audio files that are properly located in the "Audio Data" directory. The same files that it could previously handle without any issues. In one case here it was a 320 kbsp stereo .mp3 file (the .wav pendant in the Audio Data directory). I tried to replace it with an identical .wav version - but no luck. Still hanging on opening when "Loading Audio Regions ..." Waiting for half an hour did not help, and the Windows process list does not show any cpu load on Cakewalk, neither do the Windows system logs show any event entry that could fit to the situation. Neither is an antivirus solution blocking file access. All files associated with the project have been manually tested here, and all are fully accessible through other software.
For Cakewalk however, it was impossible to open that project, so I was ready to accept the worst case that 160 work hours where flushed.
The only way to being able to access the Cakewalk project file again was to remove the references .mp3 (.wav respectively) from the "Audio Data" directory. Now when I open the project, Cakewalk does not find the file and asks for a replacement, which is great, because at least we now have a chance to open the project. I then clicked "skip" in the dialog where Cakewalk asks you to replace the missing file and - voila - the project could be opened. Of course without the problematic audio file, but at least the rest of the project became available again.
So the "solution" is to remove one ore more problematic audio files from the Audio Data directory and then open the project without the files, then import new versions of the same files into the project again and save it under a new name.