I've got a problem with upgrading Moodle 1.9.10+ (Build: 20110119) to Moodle 2.0.1.
My Moodle instalation is based on windows environment (PHP 5.3.5 and Mysql 5.1.53-community, IIS 7.5).
During the upgrade I get the following exception:

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Unknown exception related to local files (Invalid contextid)

Stack trace:

line 724 of \lib\filestorage\file_storage.php: file_exception thrown
line 267 of \lib\db\upgradelib.php: call to file_storage->create_file_from_pathname()
line 4764 of \lib\db\upgrade.php: call to upgrade_migrate_group_icons()
line 1360 of \lib\upgradelib.php: call to xmldb_main_upgrade()
line 252 of \admin\index.php: call to upgrade_core()
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As I commented out the line which throws the exception,
another exception was fired, but this time it has more meaningful content:

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Can not create file "/group/icon/56///f1.jpg"
Stack trace:

line 793 of \lib\filestorage\file_storage.php: stored_file_creation_exception thrown
line 267 of \lib\db\upgradelib.php: call to file_storage->create_file_from_pathname()
line 4764 of \lib\db\upgrade.php: call to upgrade_migrate_group_icons()
line 1360 of \lib\upgradelib.php: call to xmldb_main_upgrade()
line 252 of \admin\index.php: call to upgrade_core()
=======================================================

please, check out the file name above - it is clearly wrong.