[deback]

Calibre 3.9 was working fine for an hour or so, until after I ran Check Book and fixed some errors manually. Then I had the software automatically fix the remaining 89 errors under Check Book (89 mimetype errors in content.opf). After an hour or more of waiting and looking at "Running checks, please wait" without completing the task, I finally realized I could click on Check Book again, so I did that and received this error message (I actually had received that error msg when I first clicked on fix errors but was too tired to deal with it...and it looked like it was still working, so I just kept waiting).

calibre, version 3.9.0
ERROR: Unhandled exception: <b>error</b>:can't start new thread

calibre 3.9 embedded-python: True is64bit: False
Windows-10-10.0.16299-SP0 Windows ('32bit', 'WindowsPE')
('Windows', '10', '10.0.16299')
Python 2.7.12+
Windows: ('10', '10.0.16299', 'SP0', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 4) && EpubSplit (2, 2, 1) && Find Duplicates (1, 6, 3) && EpubMerge (2, 1, 1)
Traceback (most recent call last):
File "site-packages\calibre\gui2\tweak_book\boss.py", line 71, in ans
File "site-packages\calibre\gui2\tweak_book\boss.py", line 1242, in check_requested
File "site-packages\calibre\gui2\tweak_book\check.py", line 210, in run_checks
File "site-packages\calibre\ebooks\oeb\polish\check\main.py", line 44, in run_checks
File "site-packages\calibre\ebooks\oeb\polish\check\base.py", line 50, in run_checkers
File "multiprocessing\pool.py", line 727, in __init__
File "multiprocessing\pool.py", line 159, in __init__
File "multiprocessing\pool.py", line 223, in _repopulate_pool
File "multiprocessing\dummy\__init__.py", line 75, in start
File "threading.py", line 736, in start
error: can't start new thread

Then I exited without saving and received the same error message as above. The program closed out, and now I'm going to try again with 3.9.