I’ll start with explaining the cause of the problem. If you don’t need this, you can skip the padded paragraphs.
You have probably already heard about issues with weird symbols in iPhones, that led to freeze or reboot? Something like these can also be experienced on Redmine instances, that were installed some time ago. iPhones crash because they fail to render the weird symbols and such Redmine instances give errors because they fail to save corresponding symbols to the database. Yes, the problem is actually in the underlying MySQL database, not in Redmine. In iPhones this happens with some unusual 4-byte unicode symbols and in Redmine/MySQL this can happen with any 4-byte symbol. Thus, you can experience this issue in Redmine with emoji, some Chinese, Japanese and Korean characters.