I had this issue also on at least 2 occasions.

@phlegethoth same exact thing happened to me...with the exact same chest! I got it to work by going to another chest and then the raider signaled me to come help him open it.

@sdcsuxd Same here. Sometimes they won't help force open doors either.

Got the same issue (PS4). Looks like the same chest as well.
I took a video, I can upload it if needed but it's pretty much the same as OP's.

@ubi-swaggins I have only begun raids from the longship

@ubi-swaggins
It doesn't matter -- I (and other posters) have had this bug happen many, many times during raids. You can start the raid from the longship, you can ride/run there manually and blow your horn, same result. It's a problem with the companion/raider AI, and the game's raid programming in general, as I've had other issues during raids as well: CTDs, interaction prompts missing, enemies and allies staring at each other and not fighting for seconds at a time, and so forth and so on.

Only workaround is just to clear area of enemies, then run around a bit and try again. Failing that, reload earlier save and redo the raid.

@iambadloveme Generally. Reload. Rerun the raid from scratch because you can't save during. Hope that someone doesn't stand in front of the door glitching out (which is one of the things causing this bug). If they don't, you get to finish the raid. If they glitch in front of the door again, or people just decide not to help you open the chest; you reload the entire raid for a third or fourth time (I'm currently on four this time) and hope that you get to finish the raid.

Up to five attempts. Last chest each time.
Not the same last chest mind you

Same location and same issue.

While raiding, NPCs will occasionally become unresponsive and will not help to open doors or chests. They will run back and forth randomly doing nothing, but will not help. Sometimes going to a different chest will fix it. Other times you must leave the area entirely and come back if the chest is the last one remaining during a raid.

Video example:

During raids, NPCs can become stuck clipped inside of chests making it impossible to open them until the NPC is knocked away from the. This will go on indefinitely regardless of other NPCs nearby or when using the chest yourself. The only way to resolve the issue is by shooting or hitting the NPC out of the chest somehow.

Video example:

Having problems with chests you're supposed to loot bugging on me. I and a fellow raider open a chest but the lid doesn't actually move, the chest remains closed and I get no loot, however the game tells me I still need to loot that chest. This forces you to reload an earlier save because it becomes impossible to finish that raid. Also had a crash during a raid.

@longjohn119
I don't know why I wasted my time providing videos .... They always ask me for a video but when I supply one it gets completely ignored .....

When trying to open the final chest at Readingum Abbey (the one behind the breakable wall) none of my NPC raiders are able to enter the room and just run back out half-way through, making it impossible to open the chest.

Same issue. The game crashed when i opened it during a raid. I cant open it alone and I cant restart the raid.

Same issue here. Last room so reloaded previous save to do raid again...and still the crew refuses to enter the room or help. Resolved itself as mentioned above with leaving room and returning after looting elsewhere.

I noticed on the Island of Ely Monastery that if I begin my raid from the back of the monastery instead of the village, my raid would be bugged, from people not helping me to enemies standing still. I wonder if the raids still need a specific starting point or something like that.

@e1r1kther3d I think I discovered a workaround, which worked for me. Your raiders will randomly loot corpses of dead enemy soldiers, so what I did was put a bunch of them right next to the cargo crate, and it forced them to help me open it.

when opening chest during a raid my friendly npc's would bug out when they would come to help me force open the chest. I have a video of it happening