Hello, @Tobias Giesgen !
It looks like the empty provider list was caused by a missing permission which has since been added. Thank you for the update, @Tobias Giesgen !
Normally when you are creating an Azure Quantum workspace, the provider list should populate automatically so that you can enable a provider:
The Quantinuum, IonQ, and Microsoft QIO providers are automatically added to the workspace. To add another available provider, click Add on that provider's tile.