Here, too
https://www.chess.com/game/daily/273837870
Thanks for your answer. I use Firefox on an Ubuntu laptop.
I just checked it with Firefox on my mobile device. I got the same message there, too.
I only noticed this behaviour if the remaining time is "months". If it is weeks/days/hours, everything was fine.
(I guess "mos" means "months".)
Sometimes when my opponent uses vacation time, it shows that my opponent's remaining vacation time is %1$s % Mo. like in this game: https://www.chess.com/game/daily/284721188
It could be "1 month (1 Monat)". But I'm not sure.
Am I the only one with this problem?