There must have been some problem with the database after the upgrade - this time I was lucky and the database was almost empty so I solved this with episerver CLI tool.
After upgrading the solution with the latest nuget files I simply created a new database with the command:
dotnet-episerver create-cms-database *.csproj -S . -E -dn DatabaseName.Cms -v
Then I modified my connection string to the newly created database and started the solution - and after this I could use my application with the new database.
Database upgrade is set to automatic and is latest version 8011
Site won't start after upgrade from
EPiServer.CMS 12.12.0 => 12.15.1
EPiServer.CMS.Core 12.9.2 => 12.11.0
EPiServer.Framework 12.9.2 => 12.11.0
EPiServer.Framework.AspNetCore 12.9.2 => 12.11.0
Initialize action failed for Initialize on class EPiServer.Initialization.Internal.ModelSyncInitialization, EPiServer, Version=22.214.171.124, Culture=neutral, PublicKeyToken=8fe83dea738b45b7
at EPiServer.Framework.Initialization.InitializationEngine.ExecuteTransition(Boolean continueTransitions)
at EPiServer.Hosting.Internal.EPiServerFrameworkHost.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.Run(IHost host)
at ***.Program.Main(String args) in C:\***\src\***\Program.cs:line 25
Anyone has an idea what could be wrong - can't figure this one out.