Server returns error even though creation appears successful

When I create an object (of type Friendship), I run some afterSave code.
The objects are created successfully, and the business logic appears consistent when that is the case. However the server returns the following to me somewhat inconsistently:

Server reported an error: FAULT = '0' [Unable to save object - invalid data type for properties - friends. You can change the property type in developer console.] <Unable to save object - invalid data type for properties - friends. You can change the property type in developer console.> 

and also:

Server reported an error: FAULT = '0' [[Ljava.lang.String; cannot be cast to com.backendless.BackendlessUser] <[Ljava.lang.String; cannot be cast to com.backendless.BackendlessUser> 

Also at times it returns this error when trying to add a user to their own friends list:

<java.lang.RuntimeException: java.lang.RuntimeException: java.lang.StackOverflowError>

Now like I said, everything appears to be created successfully, but I consistently get these errors at times. So from my client-side it is making things confusing as I need to handle errors vs success accordingly.
And that is hard to do when things are apparently successful but it’s telling me there’s an error. So I’m wondering if maybe I am indeed doing something wrong here or if it’s potentially another issue. Thanks

Does the problem happen when the event handler runs in the Debug mode?
Does the problem happen if you disable the event handler?

Yeah, it happens in both conditions. I’ve decided to just implement what I was doing client-side instead of in Business logic since I couldn’t figure out what was wrong here

If the problem happens without business logic in place, then the root cause must be somewhere in the class definition for the object you’re saving.

Oh sorry I meant it doesn’t happen when I remove the business logic.

Should we disregard this problem since you’re implementing it differently now?

Yeah that’s fine, if it’s not a known issue or something you guys have seen before then I’ll just move on for now. Thanks