Looks like execution flow goes through different code branch for different users. In the context of one user RockPos module was probably not loaded. PHP did not know/found the constant this module defines, and the code in override failed terribly.
Let's blame RockPos module for this 🙂 All overrides should check if the module is enabled and loaded. If not, they should NOT do anything - they should fallback to overridden method. Since you are customer of RockPos please file an issue for this, and let them fix it.
Meanwhile, you should figure out why one user has different context than other. I would guess this to be related to Customer Groups -- modules can be disabled per group, so check this settings. But of course, it can be different issues