[Baertram]

You did some PostHooks here by calling the original function before your new code, but you overwrote the inventory functions during inventory build up as it seems.
OldOrigFunc = OrigFunc
OrigFunc = First OldOrigFunc, then your code
-> During inventory build up -> code get's tainted.

What I understood (thanks to sirinsidiator):
All addon code altering inventory stuff until the inventory is fully build (and the secure code was finished) will taint the code and issue these error messages now.
Not sure why they changed this and how and what, but afai experienced this happens if you build your own "PostHooks" instead of using ZO_PreHook or the new SecurePostHook function!

This also could happen if your addon is changed properly and other addons hook the same functions in any old/other/now insecure way, but tainting the code. There is no/maybe no error message if you use the other addon alone. There is no error message if you use your addon alone. But there will be if you use yours with the new code and the other with the old code together...