Not sure what all control you have over how certain features are implemented, but the user chat feature is a pain to use on mobile.
For example I can type in a username so start a conversation with someone, but if I do anything else (try to select then, try to respond to a chat , etc.) it's either too cluttered to do anything (i.e. I hit "send" and the message doesn't send) or touch inputs don't do anything (i.e. cannot select a new user to chat with).
I tried seeing if there was a mobile friendly version of the site (m.forum.pso2.com) but there doesn't appear to be anything. If you'd like screenshots of the issue (because words are hard) let me know and I can post some.