The error occurs when you try to save a text (for example comment on a task), which contains Unicode Emojis - http://www.unicode.org/emoji/
You should make sure your database encoding is correctly set to utf8mb4 for emoji support.
As listed in recommended server configuration article and also in installation manual in the Easy Redmine install package:
collation_server = utf8mb4_unicode_ci character_set_server = utf8mb4
You should also check and other configuration files for inconsistencies. One thing worse than one incorrect setting is various inconsistent settings.