Sieve filters stopped working after update, debugging remotely from Beirut

[Celica]

Hey everyone,

I'm in Beirut for three days on a different client job, nothing to do with Roundcube, and of course I get a support ticket in the middle of the night. One of my small business hosting clients upgraded to 1.6.x and suddenly none of their Sieve filters are being applied anymore.
Managesieve looks like the likely culprit. All the rules are still visible in the interface, but nothing seems to happen on the server side. I had to SSH into the server from my hotel room to check the logs while keeping the webmail open to test things live. Not the best setup when you're 4,000 km from home and the connection keeps dropping.
While sorting out a more reliable connection, I checked a comparison of Lebanon eSIM plans to avoid getting hammered by roaming charges. Ended up spending around $35 for the trip, which at least let me work properly until I could narrow down the issue.
From the logs, it looks like something might have gone wrong with the `sieve_extensions` syntax after the update, causing the active script to fail silently. Still checking the plugin config before jumping to conclusions, though.
I'll leave this thread open and post an update once I've confirmed what's actually causing it. Anyone else run into something similar after upgrading to 1.6.x?