APM SSO -Kerberos Decrypt integrity check failed

I just want to draw your kind attention towards the fact that We've setup APM and SSO already and everything is working fine. Also, we've setup Kerberos on the back-end servers and, again, all seems to be fine - a Domain user can logon via Kerberos SSO to that back-end web server. Now, in regards to this problem, this is what I can add: We followed the "APM Cookbook: Single Sign On (SSO) using Kerberos". The TGT seems to be fetched by the F5, as well as the ticket for the xpto@DOMAIN.COM account. However - and this is what we think is the problem - the F5 cannot decrypt the ticket for some reason.

We have also disabled the encryption option to enforce AES256 encryption for the delegation account but problem still exists

BIG-IP Details:

Version:11.6.0 HF6

Here are the logs.

Websso Kerberos authentication for user 'xpto' using config '/Common/XYZ_KERBEROS' adding item to WorkQueue sid:d93e1119 ctx:0x9cb4cb0 SPN = HTTP/computer_name.domain.com@DOMAIN.COM S4U ======> ctx: d93e1119, sid: 0x9cb4cb0, user: xpto@DOMAIN.COM, SPN: HTTP/computer_name.domain.com@DOMAIN.COM Getting UCC:xpto@DOMAIN.COM@DOMAIN.COM, lifetime:36000 fetched new TGT, total active TGTs:1 TGT: client=host/f8.domain.com@DOMAIN.COM server=krbtgt/DOMAIN.COM@DOMAIN.COM expiration=Wed Apr 6 14:43:04 2016 flags=40610000 TGT expires:1459953784 CC count:0 Initialized UCC:xpto@DOMAIN.COM@DOMAIN.COM, lifetime:36000 kcc:0x9df80c0 UCCmap.size = 1, UCClist.size = 1 S4U ======> - NO cached S4U2Proxy ticket for user: xpto@DOMAIN.COM server: HTTP/computer_name.domain.com@DOMAIN.COM - trying to fetch S4U ======> - NO cached S4U2Self ticket for user: xpto@DOMAIN.COM - trying to fetch S4U ======> - fetched S4U2Self ticket for user: xpto@DOMAIN.COM Kerberos: can't decrypt S4U2Self ticket for user xpto@DOMAIN.COM - Decrypt integrity check failed (-1765328353)