All over sudden dovecot thinks FS is read only

[Tobi]

Hi list

I have to dovecot instances running which host my mailboxes on Centos7

dovecot.x86_64                      1:2.2.26.0-2.el7.centos
installed
dovecot-debuginfo.x86_64            1:2.2.26.0-2.el7.centos
installed
dovecot-mysql.x86_64                1:2.2.26.0-2.el7.centos
installed
dovecot-pigeonhole.x86_64           1:2.2.26.0-2.el7.centos
installed

all over sudden both dovecot "think" that the mailbox filesystem is read
only and fill the logs with

Oct 28 11:39:50 mbox1 dovecot: lmtp(tobster at brain-force.ch): Error:
ssY/MGZQ9FnoLgAAC4IaJw:2: sieve: failed to open logfile (LOGGING TO
STDERR): open(/home/vmail/brain-force.ch/tobster/.dovecot.sieve.log)
failed: Read-only file system

But a test at cli shows that I can write to the location

$ echo "test" >/home/vmail/brain-force.ch/tobster/.dovecot.test
$ cat /home/vmail/brain-force.ch/tobster/.dovecot.test
test
$ echo "test1" >/home/vmail/brain-force.ch/tobster/.dovecot.test
$ cat /home/vmail/brain-force.ch/tobster/.dovecot.test
test1

Mounts show that the FS should be rw
/dev/vda2 / ext4 rw,relatime,data=ordered 0 0

I restarted dovecot several time already. As well I restarted the
physical host on which the dovecot vms reside.

Filesystems are not full and are not running out of inodes.

Does anyone have an idea why dovecot suddenly behaves like that?

Thanks a lot for any hint
Regards

tobi