Somewhere between version 9.1.0.17754 and 9.1.0.18042 you have introduced a bug on the filtering system for the new "Add products" dialog. While it works on the first it doesn't in the latter.
According to my research, the function that creates the fetchxml query is somehow manipulating it and it duplicates orders and filters, causing sql timeouts and a 500 response.
Furthermore, if the selected view happens to have a linked entity, then the code is adding a "<" character in front of the link which completely breaks the XML, generating a 400 response.

Hope this help you!