Since Leap 15.6 is still beta or even alpha, I think we should bring version 28.0.1 from server:php:applications to Leap 15.6. I can't help with the SLES because I don't have the rights. You also have to be careful when updating. Always step by step. The specified versions in SLEs are EOL https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule