1. Make sure you are using the latest version of Ditto (2.0.2 stable or r1429 development version).
2. Try this Ditto call:
[!Ditto? &summarize=`10` &tpl=`testing_template` &tagData=`bereich` &tagDelimiter=`,` &dateSource=`pub_date` &extenders=`tagging` &sortDir=`DESC` &dateFormat=`%d. %m. %Y` !]
with this "testing_template" template:
<a href="[+url+]" title="[+pagetitle+]">[+pagetitle+]</a> Tags: [+tagLinks+] <br /><br />
Then copy and paste the output here
3. Add back parameters until the call ceases to function as desired.