Oh no no there's no limit
Could it be your browser? Did you try clearing cache or using a different browser/device?
there's no limit to marking them as already read or reading (I have somewhere between 520/530 I guess) but I also realized that they don't always show every manga that's been updated. I take it's either mangas that u haven't read in a while so the engine only shows u updates for mangas where it seems that u are interested in the continuation or the update of the manga took place after your daily notifications come in (then u won't be notified about it anymore, since the program says like okay at 12 for example I'm showing you all the mangas that have new updates today and then one manga is being updated on the same day at maybe 5 pm and the system's just like nah u uploaded too late today now I won't show that update to your followers hehe).. something like that.. (that is how I experienced it at the least)
Is there a limit to how many manga you can mark as reading at once because the moment I mark something else I dont get update info on other stuff and then I go back to them and they are no longer marked and it's really upsetting.