After a couple of minutes Opus shuts down, throwing some "integrity check" error. Is there a way to fix this check sum (?) related issue?
The last time I started getting integrity check errors, I reinstalled DOpus on top of existing installation. Things have been fine since then.
hmm, the 9.1.3.5 OI version runs fine, so i reinstalled that. Most changes apply to Windows 7 anyway, i´m still on a XP machine. But plan to update soon. 
possible reason: I use a special Autohotkey exe for some special command, which has to be in the GPSoft\Directory Opus folder on C: in order to function. Maybe this is recognized as an alien file?
It sounds like your install is corrupted.
Opus only cares about its own DLL/EXE files so your extra Autohotkey EXE won't cause any problems unless you've replaced one of the Opus ones with it. (i.e. Don't rename any of the Opus exes.)
I'd check the digital signature on the installer you downloaded and then run it again.
If the problem remains, check the digital signatures on the EXE and DLL files directly below the Opus program files folder. (Ignore the sub-folders.) Within the main folder, the only DLL/EXE files which shouldn't be signed are libeay32.dll and ssleay32.dll.
Digital signatures can be checked via the standard Properties dialog for each file.
Note that just looking at the Properties dialog's Digital Signatures tab only tells you that a signature exists and not that it is valid. You have to click the Details button to actually verify the signature:
If your installer's signature is invalid, download it again and check the new copy is okay. (Clear the browser cache if it is still wrong.)
If the installer signature is valid but the DLL/EXE signatures are invalid after installation, I would do a virus scan as something may be modifying DLL/EXE files on your system. (It's also possible the HDD is corrupting data, so a disk error-check (open Properties for the drive-letter and use the Tools tab) and/or SMART status-check may be a good idea.)
Thank you Leo, i have made a mistake & installed it from my download folder, which sort of belongs to Sandboxie & isn´t the right place to install from. I simply overlooked that. Normally i move all items to the
appropriate folder. 
After having the same Installer moved to an ordinary folder, the installation went through. Sorry!!