After working off and on for over a year with the same problem as
yours, Nils, I now have found a solution. None of the HP provided
solutions (e.g., safeboot and re-install) worked for me.
I disabled Microsoft's standard service called "Windows Image
Acquisition (WIA)", with the intent of eliminating any interference it
may be causing. I did this by: clicking on Start, Run, entering
"msconfig" and clicking OK. On System Configuration Utility window,
Services Tab, enable all services and then uncheck "Windows Image
Acquisition (WIA)" which is found quite far down in the list of
services. This un-checking automatically causes a change on the
General Tab of this window from Normal Startup to Selective Startup,
with a green box (rather than a green check) in the Load System
Services item (and green checks in all the other items under Selective
Startup). Then restarted system. After startup, a window appeared
that said something like "Check here to not show and to not start up
System Configuration Utility at logon". Since this window continued
to appear at every startup, I checked the box on this window, and all
still worked correctly. [It appears that "Windows Image Acquisition
(WIA)" has no dependencies, which I confirmed by: Control Panel,
Administrative Tools, Services, right click "Windows Image Acquisition
(WIA)", properties, Dependencies tab: 'The following system components
depend on this service' <No dependencies>.]
Removing the Windows Image Acquisition (WIA) service has the following
(to me, insignificant) unintended results: In Control Panel, Scanners
and Cameras – no devices are listed (as they had been otherwise).
Under Start, right click "My Computer", properties, hardware, device
manager, imaging devices – my imaging devices show normally. A second
unintended result: the two buttons on the front of the scanner do not
start an operation. Operations can only be started from the PC
programs (scan or copy).
But the two programs, "HP PrecisionScan LT" (scan) and "HP ScanJet
Copy Utility" (copy), work!
Hope this helps, and good luck.
Don Rain