Pokerstars say Black-Screen Bug is fixed - Works for me again
TL;DR: I had (eventually) replies from the Help Support, Twitter StarsSupport & the official Twitter account within 24 hours, some replying faster than other. Bug now fixed and expiry problem solved.
I messaged Support to no reply for a few hours. I then messaged Stars Support on Twitter and saw 5+ people h aving the same all platforms not working issue. Stars Support Twitter didn't reply until the next day. In desperation I messaged the official Pokerstars Twitter account and re-did the in-software Help adding a screen recording as a document. Help replied INSTANTLY and told me they were aware of the issue, were working hard on it and to try again before I went to sleep that night.
I did, it still didn't work and let them know. The Official Pokerstars Twitter account then replied and went into my account to open the chest before it expired (either the chest gave me only cash or they gave me the equivalent in cash). The next day I had replies saying bug fixed to both my Help requests and a reply saying try again from Stars Support Twitter. Chests have worked for me since.
From this seems like adding proof/documents to a help request gets it seen earlier maybe?