StartCoroutine not working for unknown reason

Well I know unity, C# and have been working on unity for several years but something very weird is going on with an attempt I make to call a coroutine.

 StartCoroutine("HandleNetworkWarnings");

is called within a case statement.

 private IEnumerator HandleNetworkWarnings()
 {
 ...
 }

is the coroutine.

The coroutine does not work and instead the log just prints the name of the coroutine in red to the console with the stack trace:

 UnityEngine.MonoBehaviour:StartCoroutine(String, Object)
 UnityEngine.MonoBehaviour:StartCoroutine(String) (at C:\BuildAgent\work\842f9557127e852\Runtime\ExportGenerated\Editor\UnityEngineMonoBehaviour.cs:74)
 GameService:HandleNetworkWarnings(String) (at Assets\Scripts\NewScripts\BaseClasses\GameService.cs:766)
 GameService:OnConnectionStatusHandler(String) (at Assets\Scripts\NewScripts\BaseClasses\GameService.cs:734)
 XServiceListener:StatusCallback(String) (at Assets\Scripts\NewScripts\BaseClasses\XServiceListener.cs:109)
 PhotonListener:PeerStatusCallback(StatusCode) (at Assets\Scripts\NewScripts\ConnectionService\PhotonListener.cs:62)
 ExitGames.Client.Photon.EnetPeer:queueOutgoingAcknowledgement(NCommand)
 ExitGames.Client.Photon.EnetPeer:receiveIncomingCommands(Byte[])

I call other coroutines in the parallel case statement and it works fine.

I am wondering whether I stumbled upon some unity bug caused here. Note that the class is abstract, but that caused no issues until this one. Also, the call is made within a callback which is called by the photon listener (3rd party multiplayer solution).