I had the same problem with this Docker as a few other people have had, but that I've not seen a definitive solution for, and I thought I'd document the best solution I've come up with. I was having an issue where the Docker would start, but the web UI would fail to become accessible. Looking at the log files, the issue was with MongoDB, which was giving the error:
Fri Aug 12 18:22:34.017 [initandlisten] LogFile::synchronousAppend failed with 8192 bytes unwritten out of 8192 bytes; b=0x35d4000 errno:22 Invalid argument
Fri Aug 12 18:22:34.017 [initandlisten] Fatal Assertion 13515
This also resulted in the following errors appearing in the unRAID system log:
Aug 12 18:22:34 fileserver shfs/user: shfs_write: write: (22) Invalid argument
As best as I can tell, the issue is that the MongoDB service is trying to write data to the mapped folder, but the operation it's trying to perform is unsupported by the unRAID file system. (This may be a completely incorrect analysis!)
I believe some people have mitigated this by having the docker data stored on an SSD instead of on the main array, but I haven't yet added an SSD to my setup, so I can only store the data on the main array. However, by changing the config path mapping from /mnt/user/appdata/unifi to /mnt/disk1/appdata/unifi, I was able to successfully start the docker. I guess that because MongoDB is now directly accessing the ext4 filesystem on the drive, it can perform whatever operation it's trying to do successfully.
This might be worth documenting for others who don't have an SSD and map the config path directly to the main array. I've seen a few reports of this error in this thread and in some of the other Unifi docker support threads (it doesn't appear to be specific to the LinuxServer image, but to any Docker image that runs MongoDB and maps its folders).
