Yesterday I did some more testings on the allocated maximum space of the vssadmin. Finally I was able to make restore points work with a max allocated size of +/- 50GB. This means that Windows 7 in my case doesn't accept to work with the shadowspace smaller than the HDD free space. I have now between 50 and 60GB free.
When I play with the slider in the system protection tab I have started the max allocated usage at 100% (93,3GB) and brought it gently down to 75GB, 60GB, till 50GB everything went smooth. As soon as I tried 40GB or even 25GB, 12GB and 4GB the snapshot area got flushed with the eventlog errors as usual.
I noticed that there is a big difference in the used SR space depending on the max allocated you reserve. With 4GB the Restore ponts increase in chunks of 100/300MB but with the max allocated 100% it becomes chunks of 1GB. I don't understand this difference. The restore points are of the same capacity but take more space. For seven RP's Windows needed 11.22GB!
What I also experienced is that sometimes after a flush the used SR space is not released immediately although the system says that there are no RP's left. When I then made the next RP, it took the next shadowcopy number instead of starting at copy 1. (See "Vssadmin list shadows" where each shadow is named "Harddiskvolumeshadowcopy1", 2, 3 etc ...). The HDD free capacity is also not decreasing. You have to reboot with the flush occuring the next time to see the freed diskspace.
Conclusion: if I dare to set the maximum allocated usage to lower than the current free HDD space Windows flushes the SR area. But it can happen that a 3 or 4 Gb doesn't make a difference.
I can of course not accept this because I need to work with only 15 to 11GB free space.
Is there somewhere a note that says that restore points do not work with less than 15GB free HDD space. If so I definitly will have to do a fresh windows installation and by a new laptop.