This one happens in both Forge and Fabric with a dedicated server on startup, though it's specific to Cutting Edge Builds (probably anything with the changes from 41dee onward)?

Some of the logs point to BlockEntityStoredPlayerImpetus.java loading ClientLevel on the server-side, but oddly enough releases before then -- even just going to f0b85 -- don't seem to show the same issue, and still have the same references to ClientLevel and are still getting registered in the same phase.

Example crash logs.
crash_fabric_remapped.log
latest_fabric_unmapped.log
latest_forge_unmapped.log

Looks like specifically the addition of the calls to CreativeItemUnlocker.infiniteMedia() in BlockEntityAbstractImpetus.java that triggered it, but since none of the ClientLevel calls need or benefit from client-specific information, just moving them all to the more generic Level seems a more robust answer.

The commit seems to be working fine from a quick test in Fabric and Forge.

Separately, the previous impetus checks didn't let you insert a Media Cube (maybe specific to Fabric?) because the Media Cube isn't valid as a source for making phials. I just added an extra || check, but I'm less sure that's the right solution.