Hello @andreas _von lochow,
There's an easy way around to remove all the special prices from all products. You don't actually need to run any script. Just go to the administration -> catalog -> manage products and click on select all at the top left of the grid view. Then, in actions dropdown on the top-right, select Update Attributes and press Submit.
See the screenshot below:
Now, in the Update Attribute Screen, scroll down to Special Price field, and check the Change checkbox. DO NOT put anything in the text box. Click on save.
See the screenshot below.
Reindex Flat data and It might take a while if you have many products, but it will work without disturbing your database. Once done, go to Index Management and Re-index everything.
If my answer is useful, please Accept as Solution & give Kudos