XF 1.5 User Tagging Problem



Since I moved the my XenForo site to another server I have major user tagging (mention) problems in posts. It is not always but frequently happening. I can't figure out what the problem exactly is but I have figured out some example.

Whenever I use the "İ" character as a first character of message. Tagging is not happening.

For example:

İstanbul'da sonbahar @username => Problem, not rendering
Istanbul'da sonbahar @username => No problem

I tried disable all add-ons but that is not helped.

Unlike previous server, the new server has PHP7 and nginx. My XenForo version is 1.5.11

Please help me. My users are very annoyed with this problem.