Perhaps it would help if you had a different litter box for each cat. Since they are both male, and the second male is becoming an adult (they can get neutered at 6 months, generally, so I assume they are sexually mature) perhaps there is some kind of angst with the older cat not wanting the “new guy” messing in his box. He probably wouldn’t have felt threatened by it when the new guy was a kitten, but now he’s probably leaving those “boy cat” scents as well.
The males are a little on the territorial side and also tend to have a definite pecking order, so Gilbert may just be asserting his authority now that Jakob is getting bigger, trying to hang on to his place in the hierarchy.
Are you planning to neuter them? That may take care of some of the problem, although depending on the personality of the cats, they may scrap from time to time anyway.
I wouldn’t be surprised if there is a connection with your cycle … they can smell the difference in your body odor, not that most *humans* would notice, but it may set off some kind of competition with them. I have known dogs with that problem :-)