Thank you Chris, it's resolved now, see below.

Am 25.01.21 um 23:47 schrieb Chris Murphy:

On Sat, Jan 23, 2021 at 7:50 AM Jakob Schöttl [off-list ref] wrote:

quoted

Hi,

In short:
When mounting a second subvolume from a pool, I get this error:
"mount: /mnt: wrong fs type, bad option, bad superblock on /dev/sda,
missing code page or helper program, or other."
dmesg | grep BTRFS only shows this error:
info (device sda): disk space caching is enabled
error (device sda): Remounting read-write after error is not allowed

It went read-only before this because it's confused. You need to
unmount it before it can be mounted rw. In some cases a reboot is
needed.

Oh, I didn't notice that the pool was already mounted (via fstab).
The filesystem where out of space and I had to resize both disks 
separately. And I had to mount with -o skip_balance for that. Now it 
works again.

quoted

What happened:

In my RAID1 pool with two disk, I successfully replaced one disk with

btrfs replace start 2 /dev/sdx

After that, I mounted the pool and did

I don't understand this sequence. In order to do a replace, the file
system is already mounted.

That was, what I did before my actual problem occurred. But it's 
resolved now.

quoted

btrfs fi show /mnt

which showed WARNINGs about
"filesystems with multiple block group profiles detected"
(don't remember exactly)

I thought it is a good idea to do

btrfs balance start /mnt

which finished without errors.

Balance alone does not convert block groups to a new profile. You have
to explicitly select a conversion filter, e.g.

btrfs balance start -dconvert=raid1,soft -mconvert=raid1,soft /mnt

I didn't want to convert to a new profile. I thought btrfs replace 
automatically uses the same profile as the pool?

Regards, Jakob