I don't know of any issues we have with exporting SmartLists to Excel. To verify, if you haven't already, see if you get the same message/error when exporting our default SmartList reports like Accounts or something like that.
You also mentioned this customer uses GP via Remote App, try removing Remote App from the equation and test, whether just using Remote Desktop Connection to connect to the server that GP is installed onto or install GP on another machine or SQL Server and have the user test.
We sometimes see behavior when going through other technologies like Citrix, RemoteApp, GP as a published app, etc., so that we try treat them like third party products where we try to remove them from the testing to make sure they're not part of the issue being seen.
Another test you can do is, on one machine where GP is installed, have all users log out of GP and then rename the GP directory from something like this >> C:\Program Files (x86)\Microsoft Dynamics\GP2018\ to be something like this >> C:\Program Files (x86)\Microsoft Dynamics\GP2018OLD\
Then in Control Panel > Programs and Features, run a Repair against the GP install which will completely re-create the GP directory under the original name, minus any third party products, customizations nor forms/reports dictionary files.
Launch GP from this 'new' GP directory and login as 'sa'. See if you can now export our default SmartLists to Excel or not.
--if you do not get the error, then we'd want to add back any forms/reports dictionary files first and test again. Then if still no error, add back any third parties or customizations, one by one, testing after each one is added, to make sure you can still export SmartLists to Excel or not. When the error returns, then that is potentially the cause of the issue being seen.
Let us know if you have any questions.
Thanks,
