When trying to save a significant large Update Set with more than 100K update records as a local XML file using the 'Export to XML' UI Action on the Update Set [sys_update_set] table, the page loads a blank page or re-directs to the default page without exporting the XML file.
The issue is caused by the UI Transaction quota rule which is by default set to 5 minutes. More information about this quota rule can be found in the following documentation chapter: Default quota rule
Increase the threshold of UI Transaction quota rule is one way to solve the issue BUT this impacts all UI Transactions for all users and might impact the instance performance.
A better option is to make sure that Update Sets are not too big. When having multiple Update Sets, you could make use of the Update Set Batching feature which enables you to group Update Sets together so you can preview and commit them in bulk.