This is exactly the issue I'm experiencing — but since 20–24 October 2026, it has escalated from a limit to a full lockout: the composer disappears, and conversations become read-only mid-session. Professional conversations used continuously since last year were severed with no warning, notice, or grandfathering.

I've opened a dedicated suggestion with the timeline, the unanswered questions, and proposals for relief: https://protonmail.uservoice.com/forums/932842-lumo/suggestions/51863964-composer-lockout-250-message-limit-introduced-wit

If you've been affected, please vote and comment there — the more documented reports, the harder this is to dismiss.