ERROR: scrubbing <dev> failed for device id 1: ret=-1, errno=28 (No space left on device)

[Christoph Biedl <linux-kernel.bfrz@manchmal.in-ulm.de>]

Hello,

checking all my btr file systems after seeing a lot of trouble on a
particular installation, I came across this:

# btrfs scrub start -B -c3 /mnt/schroot/
ERROR: scrubbing /mnt/schroot/ failed for device id 1: ret=-1, errno=28 (No space left on device)
scrub canceled for 89a02165-5975-46c7-8565-1247874531a2
        scrub started at Fri Dec 18 08:20:27 2015 and was aborted after 00:00:01
        total bytes scrubbed: 7.70MiB with 0 errors

btrfsck shows a *lot* of the "bad extent [458797056, 458801152), type
mismatch with chunk" messages that have been discussed here but
otherwise exits zero.

df shows 75% usage (4.4 of 6.5 Gibyte used)).

# btrfs fi df /mnt/schroot/
Data, single: total=5.17GiB, used=3.75GiB
System, DUP: total=8.00MiB, used=4.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=409.56MiB, used=143.44MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=48.00MiB, used=0.00B

This is btrfs-progs v4.3 running on kernel 4.3.3

Any idea how to proceed?

    Christoph