Thanks for getting back to me with this information.
I do have a few more questions which will help us continue isolating and working toward a solution.
To confirm, are you having any issues opening other folders or files you have saved in Files on your iOS device? Or, is this folder the only one giving you troubles?
Can you recall any specific changes being done to your iPhone when you first noticed the folder had become inaccessible?
Did you try logging into iCloud to see if the folder is located there and if it is, can it be opened successfully?
Do you still have this folder on that WeTransfer app you originally used to get it to Files?
If you do have access to the original folder and it's content on that app you mentioned, can you try moving it to your iPhone again as new? Feel free to check out Use the Files app on your iPhone, iPad, and iPod touch for more ways to add files into the Files app on your device.
I appreciate your time while working with me toward a solution.