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Hitting the maximum length and being unable to continue the chat is expected, but I would not expect the old conversation to become unreadable or open as a blank screen afterward.
If you haven’t already, I’d try first checking the same conversation from another browser/device and also from ChatGPT web rather than only the app. If it still opens blank everywhere, that sounds like a bug with loading archived/max-length conversations rather than just the normal “conversation too long” limit.
I also found this existing topic of similar issue and where OpenAI support recommends to open a support ticket.