Thank you for the question @wietse.delclef4DDTA!
From your description it sounds like there is a problem with the Cloud Models for Revit 2020 add-in.
Hopefully you've already found a solution for this, but if not (and for others encountering this behavior in the future): This could be caused by interactions with third party add-ins, or by a damaged installation of the cloud model add-in.
I would suggest starting by confirming that the ExportViewSelectorApp.addin exists in the Add-ins folder (C:\ProgramData\Autodesk\Revit\Addins\2020).
If it does, try disabling the other add-ins, using the instructions from the following article: How to disable add-ins for Revit (Make sure to keep the cloud collaboration components from the Notes section.)
If the ExportViewSelectorApp.addin is missing, or disabling other add-ins doesn’t clear the issue, then I would suggest using the info in the following article to reinstalling the cloud add-in: How to reinstall the Revit Cloud Worksharing Add-in
Note: If the ExportViewSelectorApp.addin was accidentally deleted, you could potentially grab a copy of it from another system (be careful about doing this from different builds of Revit 2020). Attaching a copy off this file from my system running Revit 2020.2.3.
Lance Coffey
Technical Support Specialist
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