I had a similar error, but not sure if this is the same as the other posts here. For quite some time, I would receive the "out of filament" message. I found the post that described turning off the detection in the advanced settings. That worked great until I wanted to load a new color. Nope, no can do. I had to effectively force the filament into place until it extruded. Posting a support ticket got me a new detector, which arrived today.
I was directed to the cap-opening video:
FLUX Delta Top Cap Assembly
Also note that the filament release lever has to be gently pushed sideways to get the cover off. Exercise caution when lifting, as it wants to grip securely, then pop up quickly. This may damage the detector cable, although that wasn't an issue for me. I would suggest to stand with your face close to the back side of the top cap. That way if it does pop up, you will likely bonk yourself in the nose with it, limiting its travel.
In my case, one of the detector wires had managed to get chewed up by the feed mechanism. The replacement detector did me no good, but I was able to repair the cable.
The "after" photo, with the detector working properly again: