From this game https://gl-studio.itch.io/liminal-state I opened one of its discussion pages, e.g. this one https://itch.io/t/4239858/what-do-i-open-it-with and then clicked the "Add to collection" button, typed a collection name (since my account is still empty), clicked "Private", and confirmed "Add to collection". The error message is probably not true, instead I believe it's a programming mistake: "We seem to be having server problems right now! Please try again later."

I clicked on the tab "Game" to get back to the main screen of this game. From there, "Add to collection" works fine. The screenshot shows a later (successful, aka failing) reproduction attempt.

Hm ... now I am getting "body: expected text between 1 and 20480 characters" error message when trying to post this. I will now delete the unimportant screenshot of the described collection creation attempt ... Does the image count for the amount of characters!? Are you counting the BASE64, by chance?