"No space left on device" error but I have at least 1.5TB available overall

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Hey there, I hope some one can answer this question for me.

Whenever I reboot I receive the following error, which is multiple lines of the same or similar output:

# cat /var/log/messages | grep space
....
Mar  2 16:05:07 fedora systemd-udevd: inotify_add_watch(9, /dev/sda, 10) failed: No space left on device
Mar  2 16:05:07 fedora systemd-udevd: inotify_add_watch(9, /dev/sda, 10) failed: No space left on device
Mar  2 18:53:39 fedora systemd[1]: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd[1]: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd[1]: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd[1]: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd: Failed to add watch on /run/systemd/ask-password: No space left on device
Mar  2 18:53:39 fedora systemd: Failed to add watch on /run/systemd/ask-password: No space left on device

When I go to check what's going on with my limited knowledge of linux, I can see that I have more than enough space on my 2TB HDD, which is formatted with one ext2 /boot/ partition, one btrfs / partition (with /home, /usr, /var, & /tmp subvolumes) and one swap partition.

$ df -T
Filesystem     Type      1K-blocks      Used  Available Use% Mounted on
devtmpfs       devtmpfs    8168344         0    8168344   0% /dev
tmpfs          tmpfs       8181592     37984    8143608   1% /dev/shm
tmpfs          tmpfs       8181592      1860    8179732   1% /run
tmpfs          tmpfs       8181592         0    8181592   0% /sys/fs/cgroup
/dev/sda2      btrfs    1936223232 293723736 1640296008  16% /
/dev/sda2      btrfs    1936223232 293723736 1640296008  16% /usr
/dev/sda2      btrfs    1936223232 293723736 1640296008  16% /home
/dev/sda2      btrfs    1936223232 293723736 1640296008  16% /var
/dev/sda2      btrfs    1936223232 293723736 1640296008  16% /tmp
/dev/sda1      ext2         495844    130296     339948  28% /boot
tmpfs          tmpfs       1636320       604    1635716   1% /run/user/1000

Can someone explain what this error means and how to go about fixing it? It's nothing urgent since I don't think it's actually effecting anything with my computer, but I'd still like to know what's going on.

Thanks a lot!

~ Ev