Unhandled Exception Error

[Mi3ke]

Hi,
I thought I had done the mobi to Nook-reader changeover of a bunch of books I have been given. After having done so I got readable translations.
Now, when I click on a book I get this "unhandled Exception" error message:

calibre, version 0.8.49
ERROR: Unhandled exception: <b>CriticalError</b>:Failed to launch worker process:
Traceback (most recent call last):
File "site-packages\calibre\utils\ipc\server.py", line 149, in do_launch
File "multiprocessing\connection.py", line 121, in accept
File "multiprocessing\connection.py", line 368, in deliver_challenge
File "site.py", line 66, in load_module
ZipImportError: can't find module hmac


Traceback (most recent call last):
File "site-packages\calibre\gui2\actions\view.py", line 201, in view_triggered
File "site-packages\calibre\gui2\actions\view.py", line 269, in _view_books
File "site-packages\calibre\gui2\actions\view.py", line 236, in _view_calibre_books
File "site-packages\calibre\gui2\actions\view.py", line 101, in view_format_by_id
File "site-packages\calibre\gui2\actions\view.py", line 132, in _view_file
File "site-packages\calibre\gui2\actions\view.py", line 120, in _launch_viewer
File "site-packages\calibre\gui2\jobs.py", line 249, in launch_gui_app
File "site-packages\calibre\utils\ipc\server.py", line 166, in run_job
File "site-packages\calibre\utils\ipc\server.py", line 139, in launch_worker
CriticalError: Failed to launch worker process:
Traceback (most recent call last):
File "site-packages\calibre\utils\ipc\server.py", line 149, in do_launch
File "multiprocessing\connection.py", line 121, in accept
File "multiprocessing\connection.py", line 368, in deliver_challenge
File "site.py", line 66, in load_module
ZipImportError: can't find module hmac

I would really like to know what I can do to get back to reading once again. If this is not the correct forum, please give me a hint to get to the one that is. Thanks!