Cannot resize filesystem: not enough free space

["Jakob Schöttl" <jschoett@gmail.com>]

Help please, increasing the filesystem size doesn't work.

When mounting my btrfs filesystem, I had errors saying, "no space 
left
on device". Now I managed to mount the filesystem with -o 
skip_balance but:

# btrfs fi df /mnt
Data, RAID1: total=147.04GiB, used=147.02GiB
System, RAID1: total=8.00MiB, used=48.00KiB
Metadata, RAID1: total=1.00GiB, used=458.84MiB
GlobalReserve, single: total=181.53MiB, used=0.00B

It is full and resize doesn't work although both block devices sda 
and
sdb have more 250 GB and more nominal capacity (I don't have 
partitions,
btrfs is directly on sda and sdb):

# fdisk -l /dev/sd{a,b}*
Disk /dev/sda: 232.89 GiB, 250059350016 bytes, 488397168 sectors
[...]
Disk /dev/sdb: 465.76 GiB, 500107862016 bytes, 976773168 sectors
[...]

I tried:

# btrfs fi resize 230G /mnt
runs without errors but has no effect

# btrfs fi resize max /mnt
runs without errors but has no effect

# btrfs fi resize +1G /mnt
ERROR: unable to resize '/mnt': no enough free space

Any ideas? Thank you!