Custom Add-in only half works on 4 out of 5 computers?

Good morning!

I've got a weird one here.
Hoping someone has an answer or an idea.

I have a custom built Excel Add-in that has some 28 functions on the ribbon, each one unique, and performs different tasks.
Some are as simple as opening a windows folders, some are as complex as running 8 macros from different workbooks.

Whats very strange though, is that out of the 5 computers in one of our nests at work, the add-in works flawlessly on 4 of them, without a single hiccup, but with the 5th computer, only half the macros work.

One error reports "Cannot see invisible ranges", or something along those lines, which is ridiculous, because it opens up the workbook (you can see it in the background when the code bugs), and you can even see the range.
Another error reports "Data out of range", which is one I'm familiar with, but should not apply here, because again, you can see the range clearly if you open the workbook up after closing the bug error message.

Whats even weirder, the more complicated functions that dive into other workbooks and call their macros, function, while one of the simple data-pull macros don't.

Anyone have any idea of where I can start looking into this for possible fixes?

Info:
All computers are on the same network.
All computers running same version of windows, and excel.
Macros have been fully enabled and ActiveX fully enabled.
Add-in still works fully on the other computers, cannot replicate errors on other computers.

Any help is appreciated!