Error "Could not find a valid session key" after contributions are made
This is an issue documented here: https://lab.civicrm.org/dev/core/-/issues/1997  Fixes have been merged into 5.30 branch https://github.com/civicrm/civicrm-core/pull/18525 and https://github.com/civicrm/civicrm-core/pull/18534
So you can apply the patches, or wait until the 5.30 release in early October.