Hi GregKops!
According to your description, can we understand that this issue is only happening in VPN network?
In this case, it recommends you first check this
guidance to see if it can troubleshoot your issue.
Also you can try to run the command “Get-CsConferencingCinfiguration” to make sure each of port range for client traffic type is unique. For example, you might
configure the port ranges like this:

To my knowledge, the most common file transfer method is through Instant Message(or “P2P File Transfer”).The following picture can be used to quickly reference which types of modalities support which of the two communication models.

Besides, please check if there is any related error message in Front end server event viewer.
Best Regards,
Jimmy Yang
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