The problem was solved in a russian forum
by Vladimir M. (just search for my username "SonyReaderT1" in the thread):
ReLaunch expects the SD cards in the folders "/sdcard" and "/extsd", whereas on the Sony the corresponding folders are "/mnt/sdcard" and "/mnt/extsd". So in case anybody has the same problem with ReLaunch (BTW: I had the problem only with PDFs on the external SD card!):
- You have to go in Relaunch (version 1.3.8) to Settings->Advanced Settings->General Settings->Start directory(s);
- change the entries for the start-directories to "/mnt/sdcard" and "/mnt/extsd" and save the settings.
- Go back to the file manager/launcher view of ReLaunch and press and hold the "Home" button in the left upper corner.
- Select the "/mnt/extsd" folder and voila, the problem is gone. From now on you can use the intent that pops up when selecting a PDF file to open the file with the native Reader