.NET API : DropboxClient.Sharing.AddFolderMemberAsync() no more completing

The .NET SDK method calls should eventually either return a result or raise an exception (e.g., if the connection takes too long and times out), but depending on how long the calls take, it can take some time to do so and would hang in the meantime.

Like Здравко said, if there are multiple changes at the same time in the same account or shared folder, you can run in to this 'too_many_write_operations' error, which is "lock contention". That's not explicit rate limiting, but rather a result of how Dropbox works on the back-end. This is a technical inability to make a modification in the account or shared folder at the time of the API call. This applies to all account types. This error indicates that there was simultaneous activity in the account or shared/team folder preventing your app from making the state-modifying call (e.g., adding, editing, moving, copying, sharing, or deleting files/folders) it is attempting. The simultaneous activity could be coming from your app itself, or elsewhere, e.g., from the user's desktop client. It can come from the same user, or another member of a shared folder. You can find more information about lock contention and optimizations you can make here. The app will need to be written to automatically handle this error.

In short, to avoid this error, you should avoid making multiple concurrent state modifications and use batch endpoints where possible. That won't guarantee that you won't run in to this error though, as contention can still come from other sources, so you should also implement error handling and automatic retrying as needed.

Similarly, a 504 error indicates a transient server availability issue and should be retried.

I recommend referring to the error documentation and Error Handling Guide for more information.