I cannot reply to any posts in topics I've created.

These are the two examples:
viewtopic.php?f=1310&t=3648098
viewtopic.php?f=1310&t=3689348

The image below is an example of this error. I get it on both Chrome & Firefox for Android. Logging out & back in does not solve this problem.
https://i.imgur.com/lwFxNra.jpg

[Disk4mat]

I suspect you're trying to post an emoji from your phone, which cannot be posted to this forum.