I noticed that the form with the u vowel was kept only in High German. All other germanic languages use the form with i,such as Dutch and English. Why is that?Is this the result of some sort of sound change? The existence of two alternative forms with such different vowels seems really strange to me.
The feminine suffix variants *-ingō/-ungō correspond to the masculine *-inga-/-unga-, which go back to PIE *-en-ko-/-n̥-ko- (see Krahe 1969 'Germanische Sprachwissenschaft III' p. 206). The relationship between *-en- and *-n̥- is called ablaut.