Hi wovawefo
Try prefixing the function name, the name could allready exist in another theme or plugin.
from the handbook:
All files, variables, and functions should be prefixed with a unique identifier. Prefixes prevent other plugins from overwriting your variable values or accidentally calling your functions. It will also prevent you from doing the same. Adding the initials of your plugin name or your name is a good start.
https://developer.wordpress.org/plugins/the-basics/best-practices/#prefix-everything
Can you post the complete plugin file here.