Hello Mr. Who,
Did you find any resolution for above issue? As we are also facing same issue while upgrading from Kentico 12 to 13. For me it is working fine on Local, but not on our QA servers.
I am getting error on QA "IDX10703: Cannot create a 'System.RuntimeType', key length is zero.", but on local it works fine.
I already checked below things:
1. I cleaned cookies already and Temp .net files too.
2. I've checked all CMS dlls correspond to the Kentico version.
3. I've updated .NET framework to 4.8 version.
4. I'm using System.IdentityModel.Tokens.Jwt version 184.108.40.20630.
5. I usually delete bin and obj directories before publishing the application.
Can you please let me know, if this issue got resolved for you lateron?