I wonder if it may have something to do with your Windows settings for languages. I don't know squat about that, only that some things gets changed a bit when using different ones.
Here are all the ones that I have. I keep them saved in a folder so if I forget them I just have to look. I dont use them most of the time I just use the ue for example.
ä
[alt]+132
Ä
[alt]+142
ö
[alt]+148
Ö
[alt]+153
ü
[alt]+129
Ü
[alt]+154
ß
[alt]+225
the o with the umlaut alt 148 still does not function. somebody is hiding in my PC. WARNUNG you foul feind for you will be found and eliminated in time !! 8)