Thanks for all your answers.
I was able to open it with v3.5.11 (uninstalled 4.2.8, installed 3.5.11 withou a reboot) but i did some further investigations because i noticed, that the size in memory is only 8.837.044K with 3.5.11, which is far less than over 11GB of free/avaiable memory.
By the way: The used memory size with 4.2.8 is only 7.433.952K.
First of all, let me say that because of speed reasons, i do not use a pagefile, so there is a hard limit of 16GB ram-memory if pagefile is deactivated. I noticed, that 3.5.11 starts to opening the file no mather if there is enough free memory or not. If there is enough memory, everything is fine and i can open and edit the file. If there is not enough memory, the pc is running out of memory while 3.5.11 is loading the file and pdn and a bunch of other programms stop working. So, activating pagefile with enough space is a workaround for 3.5.11 and 4.2.8., but i would not say that its a solution at all, because 4.2.8 should also be able to open the file if there is significant more memory available than the file takes.
So, it seems that 4.2.8 has some mechanism implemented, which compares the expected size in memory vs. the free memory size before it starts to load the picturedata, because the error mesage "not enough memory" appears immediately when i try to open the file. Is this correct?
If so, in general its good, that there is such a check, but if there is 11.3GB memory available and the file takes only 7.5GB in memory, this check could possibly get some improvements. In my opinion, the best method would be, if you guys could add a "sizeinmemory" flag inside of the pdn-format which is written if you save the file and checked if you try to load it.