Increment switch use

[atom]

The contents of my Mask.txt is just ?a?a?a?a?a?a?a?a?a?a?a?a?a?a?

Not sure if the word "just" fits here. That is in total 4,876,749,791,155,298,590,087,890,625 passwords.

Basically it's a number too large to fit into an unsigned 64 bit integer and that's why you receive the integer overflow detected message. So that's exactly what hashcat should do in this case. You need to rethink about your strategy.