Yes! Problem solved.
The game -also the Steam version- is Securom copyprotected.
When my retail dvd got damaged ( by taking it in and out the box at least a 1000 times ) i installed a no-dvd patch. But later i also bought a new copy of the Collector's Edition and btw later also 2 times the normal version, last copies from shops so damnd cheap. Anyway: i just uninstalled my retail game, and deleted the remaining folders (containing the files from patches 1.01 and 1.02). Result: opened Steam, right-clicked the grey Dark Messiah Might and Magic - Single Player > install game > and it goes right away! :thumbup:
So, solution: delete any no-dvd patches that Securom might notice and unmount virtual drives if you have any.
Main PC: Asus TUF Gaming 570-Pro (wi-fi) * AMD Ryzen 7 5800X * Noctua NH-D15 * Corsair Vengeance LPX 32GB * Asus TUF Radeon 6800XT * Creative AE-9PE * 2 x Samsung 980 Pro * 7 x WD Gold HDD * Corsair HX 1000 * 1 x Asus DRW-24D5MT * Dell U3010 * Windows 10 x64 *
Office PC: Asus ROG Strix X570-E * AMD Ryzen 7 3800X * Noctua NH-D15 * Corsair Vengeance LPX 32GB * MSI Radeon 5700XT * Creative Soundblaster ZxR * 2 x Corsair Force MP600 * 7 x WD Gold HDD * Corsair AX 1200W * 1 x Asus DRW-24D5MT * Dell P4317Q * Windows 10 x64 *
Old workhorse PC: * Intel i7 4790K * Noctua NH-D15S * Asus Maximus VII Hero * Corsair Force MP510 480GB M.2 SSD * 32 GB Corsair Dominator Platinum CMD32GX3M4A2133C9 * Sapphire Radeon R9 290 * 3 x Dell U2410 @ Eyefinity 5760 x 1200 * Corsair HX 1000i * 7 x WD Black / Gold HDDs * Creative Soundblaster ZxR * Asus DRW F1ST * Corsair K95 RGB * Corsair M65 PRO RGB * Steelseries 9HD * Coolermaster STC T01 * Edifier S530 * Sennheiser HD598 * Windows 10 x64 *
by taking it in and out the box at least a 1000 times 
) i installed a no-dvd patch. But later i also bought a new copy of the Collector's Edition and btw later also 2 times the normal version, last copies from shops so damnd cheap. Anyway: i just uninstalled my retail game, and deleted the remaining folders (containing the files from patches 1.01 and 1.02). Result: opened Steam, right-clicked the grey Dark Messiah Might and Magic - Single Player > install game > and it goes right away! :thumbup: 
