Yes sorry - I didn't mean for you to install the plugin again, I meant just exit and restart Calibre using the debug option. When Calibre starts up, it attempts to load all of the GUI plugins you have installed. If there are any errors, you will see that in the output. That is why a plugin "disappears" from the menu bar (or rather does not appear). And because it hasn't loaded, that is why going into Preferences->Plugins and trying to customize it you get an error dialog thrown in your face (without my helpful debug statements).
You don't get asked about which toolbar etc to install on again because as far as Calibre is concerned it has remembered your choice from last time, and as soon as it can load the plugin it will display it there
I have replicated your ZipImport error using your folder name, so I am now 100% certain that is the cause of the issue, which is way further ahead of where we were previously. The next step is how to fix it or workaround it. Note that this appears to be a Python library level (zipimporter/os.path.dirname) problem, though it needs a Python guru like Kovid to confirm I am not talking crap about that.
One definite workaround would be to change where calibre stores all your configuration information. See http://calibre-ebook.com/user_manual...ment-variables
. If you moved your current Calibre folder from C:\Documents and Settings\vlakuz\Data aplikací\calibre\ to a directory that contained only ascii non-accented characters in it, set the CALIBRE_CONFIG_DIRECTORY environment variable to that folder then it should work ok for you. You will need to reinstall any plugins again if you do that though as there paths are hard-coded in a configuration file, but you won't lose any of your other settings.
In the mean-time I can keep experimenting and perhaps Kovid may have a suggestion for an alternative fix.