For my thesis I am encountering a similar question. I think I will try to mirror the definition of usability and state that everything interfering with it is a usability problem.
For my thesis I might use something like:
"An usability issue in the context of this research is defined as any
deviation from the criteria of effectiveness, efficiency, and
satisfaction as set forth by the ISO 9241-11 standard. Specifically,
it refers to a product characteristic or feature that hinders
specified users from achieving their specified goals in a specified
context of use, thereby negatively impacting the overall user
experience."
In addition, I have found a paper which came up with an usability definition, but I will rather stick with staying as close as possible to ISO 9241-11:
Defining Usability Problems by Pavel Manakhov & Vyacheslav D. Ivanov