svn: warning: W120171: Error running context: An error occurred during SSL communication

Hello!

What is going wrong if I get
                svn: warning: W120171: Error running context: An error occurred during SSL communication
five out of six times I try to update a repository or to do some other subversion command?

(This is Tortoise SVN/64
TortoiseSVN 1.8.2, Build 24708 - 64 Bit , 2013/08/27 19:20:39
Subversion 1.8.3, -release
apr 1.4.8
apr-util 1.5.2
serf 1.3.1
OpenSSL 1.0.1e 11 Feb 2013
zlib 1.2.8

and it's command line tool

svn, version 1.8.3 (r1516576)
   compiled Aug 27 2013, 22:13:03 on x86-microsoft-windows [...]
* ra_serf : Module for accessing a repository via WebDAV protocol using serf.
  - using serf 1.3.1
  - handles 'http' scheme
  - handles 'https' scheme
on Windows 7)

The problems started yesterday (tsvn 1.8.1, about 3 out of 4 tries failed), today nothing works.
I upgraded to 1.8.3, tried both the gui and the command line version.

Where do I have to look for the cause of this failure?

Mit freundlichen Grüßen
Dr. Hartmut Niemann

Siemens AG
Infrastructure & Cities Sector
Rail Systems Division
Locomotives and Components
IC RL LOC EN LE 8
Werner-von-Siemens-Str. 67
91052 Erlangen, Deutschland
Tel: +49 9131 7-34264
Fax: +49 9131 7-26254
mailto:hartmut.niemann_at_siemens.com

Siemens Aktiengesellschaft: Vorsitzender des Aufsichtsrats: Gerhard Cromme; Vorstand: Joe Kaeser, Vorsitzender; Roland Busch, Brigitte Ederer, Klaus Helmrich, Barbara Kux, Hermann Requardt, Siegfried Russwurm, Peter Y. Solmssen, Michael Süß; Sitz der Gesellschaft: Berlin und München, Deutschland; Registergericht: Berlin Charlottenburg, HRB 12300, München, HRB 6684; WEEE-Reg.-Nr. DE 23691322
Received on 2013-09-24 16:42:42 CEST