Sounds to me like a clutch on its last leg! Although I do not own a standard Volvo, cars of my past have been standard and through those experiences I know the symptoms of a weak clutch. Often when the clutch is worn the bearing on which the clutch rides is worn as well causing enough friction to keep the gears spinning. no harm done until you try to shift when at rest, ie into reverse, then the stationary clutch and spinning gear give you a unsatisfying grind as they engage. If the clutch is new then the issue is the bearing alone. The bad news is that to replace the bearing ( Called throw out/back bearing) the clutch must be disassembled.

Good luck