I hope someone can help me with a hint on where to look and what to look after.

I'm upgrading an Umbraco 7.17.5 to Umbraco 8.1.6 and everything seems ok, except this fatal error:

The database failed to upgrade. ERROR: The database configuration failed with the following message: Input string was not in a correct format. Please check log file for additional information (can be found in '/App_Data/Logs/')

The log file dosen't show more info (what, where), so I'm totally blank.

Bellow, the entries regarding the error:

{"@t":"2022-01-03T08:40:06.1714812Z","@mt":"Installation step {Step} failed.","@l":"Error","@x":"System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Umbraco.Web.Install.InstallException: The database failed to upgrade. ERROR: The database configuration failed with the following message: Input string was not in a correct format.\n Please check log file for additional information (can be found in '/App_Data/Logs/')\r\n   at Umbraco.Web.Install.InstallSteps.DatabaseUpgradeStep.ExecuteAsync(Object model)\r\n   --- End of inner exception stack trace ---\r\n   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)\r\n   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)\r\n   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)\r\n   at Umbraco.Web.Install.Controllers.InstallApiController.<ExecuteStepAsync>d__14.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)\r\n   at Umbraco.Web.Install.Controllers.InstallApiController.<PostPerformInstall>d__11.MoveNext()","Step":"DatabaseUpgrade","SourceContext":"Umbraco.Web.Install.Controllers.InstallApiController","ProcessId":5884,"ProcessName":"w3wp","ThreadId":172,"AppDomainId":3,"AppDomainAppId":"LMW3SVC4ROOT","MachineName":"WIN10","Log4NetLevel":"ERROR","HttpRequestNumber":11,"HttpRequestId":"01322042-a023-4bbf-9a24-4342fded4c33"}
{"@t":"2022-01-03T08:40:06.1714812Z","@mt":"{EndMessage} ({Duration}ms) [Timing {TimingId}]","EndMessage":"Step completed","Duration":2082588,"TimingId":"3f3f049","SourceContext":"Umbraco.Web.Install.Controllers.InstallApiController","ProcessId":5884,"ProcessName":"w3wp","ThreadId":172,"AppDomainId":3,"AppDomainAppId":"LMW3SVC4ROOT","MachineName":"WIN10","Log4NetLevel":"INFO ","HttpRequestNumber":11,"HttpRequestId":"01322042-a023-4bbf-9a24-4342fded4c33"}
{"@t":"2022-01-03T08:40:06.1714812Z","@mt":"An error occurred during installation step {Step}","@l":"Error","@x":"System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Umbraco.Web.Install.InstallException: The database failed to upgrade. ERROR: The database configuration failed with the following message: Input string was not in a correct format.\n Please check log file for additional information (can be found in '/App_Data/Logs/')\r\n   at Umbraco.Web.Install.InstallSteps.DatabaseUpgradeStep.ExecuteAsync(Object model)\r\n   --- End of inner exception stack trace ---\r\n   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)\r\n   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)\r\n   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)\r\n   at Umbraco.Web.Install.Controllers.InstallApiController.<ExecuteStepAsync>d__14.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)\r\n   at Umbraco.Web.Install.Controllers.InstallApiController.<PostPerformInstall>d__11.MoveNext()","Step":"DatabaseUpgrade","SourceContext":"Umbraco.Web.Install.Controllers.InstallApiController","ProcessId":5884,"ProcessName":"w3wp","ThreadId":172,"AppDomainId":3,"AppDomainAppId":"LMW3SVC4ROOT","MachineName":"WIN10","Log4NetLevel":"ERROR","HttpRequestNumber":11,"HttpRequestId":"01322042-a023-4bbf-9a24-4342fded4c33"}

Any help much appreciated. Thanks