Yesterday my rep was 1'655, today it's 1'640. Now the strange thing is: It doesn't show how I lost 15 points. Any hints on how this can happen...?
You get 15 points for an accepted answer. Probably someone accepted one of your answers but later accepted a different answer. I just tested it by "unaccepting" an answer on one of my questions. The user's rep changed immediately.
Logs don't show a recalculation or any downvoting, but they do say you associated an account which granted you 100 rep. Did you perhaps misread something? You have 1740 rep now, and you posted this 2 hours ago, so I don't think the 1640-1740 difference was from upvoting, just from the added 100 rep from the association.
Sometimes the rep is just out of sync. My experience is that this occurs when some of your previously earned rep don't count anymore. Eg. the question is deleted, you delete your own answer with upvotes, OP accepts another answer etc.
If you go to https://ux.stackexchange.com/reputation, you will always see the correct rep!
At that page you can also click a recalculate button to manually recalculate you rep.