This does sound like a mechanical shutter fault rather than user error. Based on your description, the shutter blades themselves don’t appear obviously damaged, so a likely cause is that part of the shutter linkage/arm or timing mechanism is disconnected, stuck, or out of alignment.

It may be repairable by a technician who still services older film SLRs, but for a camera like the Dynax 600si it’s often not economical. Repair labor can easily exceed the value of the camera, so in practice it is usually cheaper to replace the body with another working example.

A few notes: opening the back with film loaded won’t cause this shutter problem, but it will fog exposed frames. Depth-of-field preview should mainly stop down the lens aperture; it should not be used to judge shutter operation.

So: yes, probably fixable in principle, but usually only worth it if you specifically want to keep this body and can find a repair specialist.