I managed to ‘resolve’ the issue in a couple of steps:
- Painfully go through the process of adding the mods one at a time until I find the one that blows up when loading. Remove that mod from the server.
- If I still can’t connect, unsubscribe from all my mods in steam, then reconnect to the server and subscribe again to download the mods again.
Potentially unrelated, I also played with the order of the mods, rebooted my machine about a dozen times, updated my graphics driver, tried launching Conan-sandbox directly and from steam…and god knows what else that I’m forgetting.