Thanks for the report — the traceback tells me exactly what happened, and it is not caused by your Factorio version.
This mod is a fork of Companion Drones, so it inherited the same remote interface names as the original (companion_api, companion_speed, companion_remote_for_jetpack). Factorio allows only one mod to own an interface name in a game session, so if another mod from the same family is enabled at the same time — the original Companion Drones, companion-drones-mjlfix, or another fork — whichever one loads second aborts with exactly this message and the save refuses to open.
Workaround right now: keep only one of them enabled. Disable the other Companion Drones mod (or this one) and your save will load again. Nothing in the save itself is damaged.
The actual fix: version 2.1.1 registers its own unique names (bodyguard-companion-api, bodyguard-companion-speed) and claims the legacy names only while they are still free, so both mods can be installed side by side without crashing. One caveat: 2.1.1 targets Factorio 2.1, so while you are on Factorio 2.0 the portal will keep serving you 2.0.4 — until you update the game, please use the workaround above.
If you can, please send me these files so I can confirm the cause instead of guessing:
factorio-current.log — the most useful one. It lists every enabled mod with its version, which tells me immediately whether a second Companion mod is the culprit.
mods/mod-list.json — tiny file, same information in short form.
The save file itself, if you are able to share it — optional, since this crash happens while mods load rather than while reading the save, but it lets me reproduce your exact setup.
Where to find them:
Windows: %APPDATA%\Factorio\ (log and mods\ are here, saves in saves)
Linux: ~/.factorio/
macOS: ~/Library/Application Support/factorio/
