There is nothing that anyone can do to help you until you provide some information. Please provide an example/screenshot or link to a post or a way to reproduce your issue.
Is this happening to all of your users?
Is your user confused that "BBCode Only" is the default entry excepted for users?
I post all the time from all of my mobile devices, without any issue
I'm at a loss as to what you are saying or asking to be fixed.
Is your user attempting to post characters that are not supported or cannot be interpreted correctly by your server? Meaning...
Does your database Connection Character Set match its Collation? Have you recently made changes to the connector without updating the database collation?
Control Panel > Paths & Database: Database > Set Default Client Encoding to UTF8https://dev.mysql.com/doc/refman/5.7/en/charset-connection.html