upgrade from 11.2.1 to 11.5.1 failed

but how to know what is the real cause? What about this message: (expired or digital signature does not match contents).?

i think that message is not really useful. normally if restoring ucs fails, running "tmsh load sys config" would give useful error message (although restoring fails, config file is in /config already. so, when loading config, it will show an error if had) and we can start from it i.e. edit config file directly and re-load the config again.