Too many dots in Safari password fields
Found it, my fault. I had recently defined a system-wide shortcut associating the symbol "•" to another string. Worst idea ever! As soon as a dot appeared, it was expanded to several characters, all replaced by dots, and so on. Since I usually use 1Password, I only hit the issue a few days after the fact.

Now there's something on the internet in case somebody makes the same mistake :)