Nano V3 values "overwritten" randomly
I found the problem. I could see that the false value is the character code of the minute I am displaying. I display hours and minutes and there was a declaration that was 3 characters short for the string I got as a result for hours added to minutes. Yes, it was memory corruption. So be very careful to dimension enough places for your C-strings including 1 extra for 0. If you are not so short on RAM then a few more characters will do no harm, the end of the string will be the character before the 0 in memory.
When strange things happen start looking at this matter!