Have the same issue with 2 of my servers.
but i also have 8 other servers that are working fine.
Mine happened after reboot of the cimc/device (UCSE 180d/m3 on ISR4351)
TACACS server logs has the same error:
"TACACS: Invalid TACACS+ request packet - possibly mismatched Shared Secrets"
but it is working x10 logins ago, so it is impossible that shared secrets are mismatched.
disabling of tacacs on ISE does not working. using local admin/password does not work. both ssh/https
Need to factory reset the UCSE via cli/console.