here's a ethereal trace of what happens between konqueror and timsieved:

"IMPLEMENTATION" "Cyrus timsieved v1.0.0" 
"SASL" "GSSAPI CRAM-MD5 DIGEST-MD5 LOGIN PLAIN" 
"SIEVE" "fileinto reject envelope vacation imapflags notify subaddress regex" 
OK 
AUTHENTICATE "DIGEST-MD5" 
{216} 
cmVhbG09ImNlbGVicmltYm9yIixub25jZT0ieEU5K09ING15YzBwMXF3WUdTT2VGVkhDd0RvN2ZiYjQ1UzA3dzhjekdIQT0iLHFvcD0iYXV0aCxhdXRoLWludCxhdXRoLWNvbmYiLGNpcGhlcj0icmM0LTQwLHJjNC01NixyYzQiLGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNz 
"Y2hhcnNldD11dGYtOCx1c2VybmFtZT0ibGVtbXkiLHJlYWxtPSJjZWxlYnJpbWJvciIsbm9uY2U9InhFOStPSDRteWMwcDFxd1lHU09lRlZIQ3dEbzdmYmI0NVMwN3c4Y3pHSEE9IixuYz0wMDAwMDAwMSxjbm9uY2U9Ik16azBNems9IixkaWdlc3QtdXJpPSJzaWV2ZS9jZWxlYnJpbWJvciIscmVzcG9uc2U9YmYwNGI4NTJhMmE5MDk2ZWM4MTJiODdjOTc0ODNlZGQscW9wPWF1dGg=" 
NO ("SASL" "authentication failure") "Authentication error" 
LOGOUT 
Ok "Logout Complete" 


and here's whats happening between sieveshell and timsieved in comparison:

"IMPLEMENTATION" "Cyrus timsieved v1.0.0"
"SASL" "GSSAPI CRAM-MD5 DIGEST-MD5 LOGIN PLAIN"
"SIEVE" "fileinto reject envelope vacation imapflags notify subaddress regex"
OK
AUTHENTICATE "LOGIN"
{12}
VXNlcm5hbWU6
{8+}
bGVtbXk=
{12}
UGFzc3dvcmQ6
{12+}
b3ZlcmtpbGw=
OK


so it seems that timsieved and kio_sieve have different meanings about how a digest-md5 auth should look like.

Can you please try kdepim/kioslaves/sieve from HEAD? We use another SASL library there, and I suspect a bug in the old library, kdesasl.

uh... sure, i could, if only i had any clue about how to do that...

Get one of the snapshots from ftp://ftp.kde.org:/pub/kde/snapshots/kdepim-*, install kdelibs-devel and compile according to the instructions on http://developer.kde.org/build/
(basically the normal ./configure; make ; make install)

make[3]: Entering directory `/usr/src/packages/BUILD/kdepim-041024/libkdepim'
o kdepimwidgets.cpp ./kdepim.widgets
make[3]: o: Command not found
make[3]: [kdepimwidgets.cpp] Error 127 (ignored)
if /bin/sh ../libtool --silent --mode=compile --tag=CXX g++ -DHAVE_CONFIG_H -I. -I. -I.. -I.. -I../libkdepim -I/opt/kde3/include -I/usr/lib/qt3/include -I/usr/X11R6/include   -DQT_THREAD_SUPPORT  -D_REENTRANT -D_FILE_OFFSET_BITS=64 -DQT_PLUGIN -Wnon-virtual-dtor -Wno-long-long -Wundef -ansi -D_XOPEN_SOURCE=500 -D_BSD_SOURCE -Wcast-align -Wconversion -Wchar-subscripts -Wall -W -Wpointer-arith -Wwrite-strings -g3 -fno-inline -O2 -g -march=i586 -mcpu=i686 -fmessage-length=0 -Wformat-security -Wmissing-format-attribute -fno-exceptions -fno-check-new -fno-common -DQT_CLEAN_NAMESPACE -DQT_NO_ASCII_CAST -DQT_NO_STL -DQT_NO_COMPAT -DQT_NO_TRANSLATION  -MT kdepimwidgets.lo -MD -MP -MF ".deps/kdepimwidgets.Tpo" -c -o kdepimwidgets.lo kdepimwidgets.cpp; \
then mv -f ".deps/kdepimwidgets.Tpo" ".deps/kdepimwidgets.Plo"; else rm -f ".deps/kdepimwidgets.Tpo"; exit 1; fi
g++: kdepimwidgets.cpp: No such file or directory
g++: no input files
make[3]: *** [kdepimwidgets.lo] Error 1
make[3]: Leaving directory `/usr/src/packages/BUILD/kdepim-041024/libkdepim'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/usr/src/packages/BUILD/kdepim-041024/libkdepim'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/usr/src/packages/BUILD/kdepim-041024'
make: *** [all] Error 2

ok, i only compiled libksieve and the sieve kioslave, and replaced the installed ones with those... now i get this:

Beim Laden von sieve://mathias@celebrimbor:2222/ ist folgender Fehler aufgetreten:


Authentifizierung fehlgeschlagen, An error occured during authentication: SASL(-1): generic failure: GSSAPI Error: Miscellaneous failure (see text) (PrivoxyWindowOpen(/tmp/krb5cc_1000): Datei oder Verzeichnis nicht gefunden) wird nicht unterstützt

Seems like something for you to figure out, Szombathelyi...

I think you have GSSAPI authentication enabled on the sieve server, but you don't have the proper credentials cache. If you're not using Kerberos, currently there are 2 solutions: setup the sieve server to not return GSSAPI as a supported mechanism, or pass the preferred mechanism to the sieve ioslave via the "sasl" metadata. However this is not possible with Konqueror (but this is exactly what KMail does). Maybe the sieve url can contain the preferred sasl mechanism, e.g sieve://sieve.host.com/path?x-mech=DIGEST-MD5?

Or another solution (I don't know if it works) to remove the cyrus-sasl GSSAPI plugin from the client machine.

Reopen it if you still have this problem in new version
Thanks