lyx disk: error: LyX could not create the temporary directory '/tmp/lyx_tmpbuf1'

[Wolfgang Engelmann]

I have saved a lyx document under a different name. I restarted lyx  
but get a lyx disk: error: LyX could not create the temporary  
directory '/tmp/lyx_tmpbuf1' (Disk is full maybe?)

Before there were in /tmp 3 tmpbuf (0, 1, 2). Now there is only this one:
lyx_tmpdir.JvcZAzSC4585

Whatever lyx file I try to load (file>recent>...  or file>open>...) I  
get always this error.

What should I do to get lyx working again?

Wolfgang