Do against the blue progress circles

Since the Dropshot update, when I play Rocket League with two local players (split screen),

• I can not see my inventory (100% of items affected)
• I do not see what items I win (drops) (100%)
• I can not trade (100%)
• I can hardly change my car in the garage (50%)

It works fine with a single player. Not problems at all then.

Of course I don't trade with my splitscreen player, because he has exactly the same items as I (same account) but I trade with someone else. I am often playing locally with 2 PCs and 3 players (you can see that from the screenshot: one PC is German, other is English). It happens on both machines, depending on which one has the splitscreen. This pretty much excludes a hard disk problem.

If it happens, here's what it looks like until I restart Rocket League.

Screenshot of inventory:

Screenshot during trade:

Screenshot during customizing:

Screenshot of drop after trade:

I have already tried:

• check the integrity of local files via Steam
• look at this Steam discussion and this one
• look at this Reddit post
• found this RL Support Request

but none of them is really helpful.

What can I do against the blue progress circles?