Unhandled exception, UnicodeDecodeError: 'utf-8' codec can't decode byte 0xfc in position 1031: invalid start byte

Hello everybody...

I am calling getmail from IMAP Grab 0.1.4 on Raspian jessie and get the 
following error:

=================================================================

Downloading mailbox "Gesendet"

Exception: please read docs/BUGS and include the following information 
in any bug report:

   getmail version 6.10
   Python version 3.4.2 (default, Sep 16 2019, 19:58:00)
[GCC 4.9.2]

Unhandled exception follows:
     File "/usr/local/bin/getmail", line 809, in main
     destination = destination_func(**destination_args)
     File 
"/usr/local/lib/python3.4/dist-packages/getmailcore/destinations.py", 
line 86, in __init__
     ConfigurableBase.__init__(self, **args)
     File 
"/usr/local/lib/python3.4/dist-packages/getmailcore/baseclasses.py", 
line 356, in __init__
     self.checkconf()
     File 
"/usr/local/lib/python3.4/dist-packages/getmailcore/baseclasses.py", 
line 365, in checkconf
     self.conf[item.name] = item.validate(self.conf)
     File 
"/usr/local/lib/python3.4/dist-packages/getmailcore/baseclasses.py", 
line 307, in validate
     first_line = f.readline()
     File "/usr/lib/python3.4/codecs.py", line 313, in decode
     (result, consumed) = self._buffer_decode(data, self.errors, final)
   UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf6 in position 
1871: invalid start byte

=====================================================================
ocbak@hugo:~ $ getmail --dump
Error: Default config/data dir "/share/ocbak/.getmail/" does not exist - 
create or specify alternate directory with --getmaildir option

=====================================================================


It happens only in 2 of 20 mail folders. Is this an Issue of IMAP Grab, 
getmail or Python itself?

What can I do?

Thanks a lot

Florian