I have the same problem, I've noticed that:

- they appear sometimes, for a period of time, I think a few hours, in almost all hosts simultaneously

- they are very difficult to decipher, but I'm pretty sure that in most cases even if typed correctly they don't work, if not after several attempts

- they occur only with jdownloader, if during the period in which they are you try with browser you will get the normal ones

No solution?