Thanks for trying that. It seems like, for some reason, the message being returned from the failed login attempt can't be converted from JSON. Are you definitely using v11.6 or higher of the LTM? One way to try and get a little more info about the error message would be to:
1) Open the file F5-LTM\Public\Invoke-RestMethodOverride.ps1 file in a text editor
2) On line 36, add a to comment out the JSON conversion and expanded property selection:
$message = $_.ErrorDetails.Message | ConvertFrom-json | Select-Object -expandproperty message
3) Add a new line below line 36 to write out the content of $message:
Maybe that will give us a better idea of what's going on.