September 2025 CU for SharePoint Server Subscription Edition included an security enhancement for the encryption algorithm used by the secure store service to save credentials.
As a side effect credentials stored with the old algorithm could no longer be decrypted.
When trying to edit an existing target application or when trying to add/update credentials the following error would occur:
Reference:
Solution
A fix for this issue has been released with March 2026 CU which allows decoding of the credentials stored with the old encryption algorithm.