Hi Kovid, thank you very much for your reply, and also for creating this awesome application!

Unfortunately, the beta you linked in  bug #1360200 does not solve the problem.

1. The traceback in my original post appears when I run calibre from the terminal without any parameters (that is, without --detach)

2. Running "calibre-debug -g 1> log.txt 2>&1" gives the following output: 
 calibre 2.0  isfrozen: True is64bit: True
Linux-3.5.0-54-generic-x86_64-with-debian-wheezy-sid Linux ('64bit', 'ELF')
('Linux', '3.5.0-54-generic', '#81~precise1-Ubuntu SMP Tue Jul 15 04:02:22 UTC 2014')
Python 2.7.8
Linux: ('debian', 'wheezy/sid', '')
Successfully initialized third party plugins: Kindle Collections && Antikvarium_hu && Libri_hu && Find Duplicates && Barnes & Noble && Moly_hu && Manage Series && Goodreads
Starting up...
Traceback (most recent call last):
  File "site-packages/calibre/gui2/ui.py", line 163, in __init__
  File "site-packages/calibre/gui2/ui.py", line 177, in init_iaction
  File "site-packages/calibre/customize/__init__.py", line 582, in load_actual_plugin
  File "importlib/__init__.py", line 37, in import_module
  File "site-packages/calibre/customize/zipplugin.py", line 179, in load_module
  File "calibre_plugins.kindle_collections.ui", line 11, in <module>
ImportError: No module named PyQt4.Qt
Traceback (most recent call last):
  File "site-packages/calibre/gui2/ui.py", line 163, in __init__
  File "site-packages/calibre/gui2/ui.py", line 177, in init_iaction
  File "site-packages/calibre/customize/__init__.py", line 582, in load_actual_plugin
  File "importlib/__init__.py", line 37, in import_module
  File "site-packages/calibre/customize/zipplugin.py", line 179, in load_module
  File "calibre_plugins.find_duplicates.action", line 11, in <module>
ImportError: No module named PyQt4.Qt
libpng warning: iCCP: Not recognizing known sRGB profile that has been edited

3. Running "calibre-debug -c "import os; os.fork() != 0 and sys.exit(0); os.setsid(); from calibre.gui2.main import main; main()""
produces: 

Traceback (most recent call last):
  File "site-packages/calibre/gui2/ui.py", line 163, in __init__
  File "site-packages/calibre/gui2/ui.py", line 177, in init_iaction
  File "site-packages/calibre/customize/__init__.py", line 582, in load_actual_plugin
  File "importlib/__init__.py", line 37, in import_module
  File "site-packages/calibre/customize/zipplugin.py", line 179, in load_module
  File "calibre_plugins.kindle_collections.ui", line 11, in <module>
ImportError: No module named PyQt4.Qt
Traceback (most recent call last):
  File "site-packages/calibre/gui2/ui.py", line 163, in __init__
  File "site-packages/calibre/gui2/ui.py", line 177, in init_iaction
  File "site-packages/calibre/customize/__init__.py", line 582, in load_actual_plugin
  File "importlib/__init__.py", line 37, in import_module
  File "site-packages/calibre/customize/zipplugin.py", line 179, in load_module
  File "calibre_plugins.find_duplicates.action", line 11, in <module>
ImportError: No module named PyQt4.Qt
libpng warning: iCCP: Not recognizing known sRGB profile that has been edited

4. Running "calibre-debug -c "from calibre.gui2 import detach_gui; detach_gui(); from calibre.gui2.main import main; main()"" gives the same traceback as in step 3.

Any ideas how I can help you to solve the issue?