since I measured a cap my multimeter has been showing the wrong values for volt. a full 1,5V AA battery shows around 6,5V and a 9V battery shows around 25~30V.
I took the meter apart but can't see any immediate damages or shorts.
Probably 'cooked' one of the input voltage divider ranging resistors, but other failure modes are possible. Time to get a new meter. The best digital multimeter have pretty good input protection circuitry up to the maximum rated input voltage. Do you have link to your specific meter?
Also, I have an older one that has the same problem (also tried measuring a cap with it..), it doesn't have a model or brand name though.
I've taken them both apart, to see if any resistors are burnt or any other damages are visible. No luck so far.