user warning after configuration and saving a jump-block

It looks like the region isn't getting set properly on the garland theme. Also looks like you're using a database prefix -- I've never tested that before. Can you test this on a site without using a database prefix? Or try a different theme when you add the jump menu?

I's no difference wether using garland or another theme.
But your prosumption is right:
by testing the module on an other site (without db prefix), the module does'nt create that warning
and the double block-entrys anymore.

Now I'm a little bit irritated.
I've tried these following steps.

I deleted all the prefixes in the db.
The result: the same warning (without prefix):
user warning: Duplicate entry 'garland-jump-menu-menu-gewerbliche-stellen-me' for key 2 query: INSERT INTO blocks (module, delta, theme, status, weight, region, custom, throttle, visibility, pages, title, cache) VALUES ('jump', 'menu-menu-gewerbliche-stellen-menue', 'garland', 0, 0, '', 0, 0, 0, '', '', 1) in /Applications/MAMP/htdocs/accu01_drupal/includes/common.inc on line 3324.

In the next step I deinstalled the module »Jump«.
Result: no warnings on admin/build/block

I installed »Jump« again (after running update).
Result: the warning is shown again.

I wonder if the block was still left in the blocks table even after you disabled the jump module. I know that there's no code in the jump module to remove any rows in the blocks table.

It works on your clean site. Great - we can get it to work on the other site too.

Try this on the site that used to have db prefix that is still broken with jump:

1. Disable jump module
2. Delete any rows in the blocks table that have 'jump' as the module
3. Re-enable the jump module

Hopefully this will fix your problem. We'll still need to figure out why it's broken on sites with prefixed tables. There are probably some missing {curly braces} around a table name in a SQL statement somewhere.