Hello Mabel,
So after some troubleshooting I figured out the problem, and it seems that it's actually in the source code of Flow itself. I'll try to explain it the best as possible...
The first action is "When and email it flagged", the default folder for this action to monitor on is the users Inbox, I'll call this the "Defualt Folder" for the rest of this post.
If I NEVER CHANGE that Deafult INBOX folder in that first action and then build the flow as needed everything works as expected. I can then share the flow whether by exporting a .zip package or adding a co-worker as an owner, or making it a team flow, and have no issues.
Now if I change the Default INBOX folder in the first action (When an Email is Flagged) from INBOX to anything else (Sent Items, Drafts, etc.) it will run perfectly fine on my account (I am the creator of the flow) but if I share it, in anyway, with anyone, the very first action will get the error above. Even if I change the Default INBOX Folder to something else and BACK TO Inbox it gets an error when it is shared. But again I can change it to anything on and it runs fine on my account, but only my account. Its almost like whatever the code is changed in the flow when switching that folder writes itself in such a way that when shared it breaks. (I have not had time to delve into what that code is as of yet)
Here's where it gets really weird. Once I share the working flow (with the Default INBOX Folder NEVER changed) and another user makes a copy on their account then changes the folder themselves, it works fine.
TL;DR: you cant change the first action folder and share the finished flow. It breaks.
