Error activating plugin

I get the following error trying to activate WordFence:
Plugin could not be activated because it triggered a fatal error.

This is for version 6.1.2.

I tried installing an older version (5.3.12) and it installed and activated but then I got the white screen of death. Debugging gave me the following:

[14-Apr-2016 18:38:49] PHP Warning: require_once(lib/wordfenceConstants.php): failed to open stream: No such file or directory in E:\Inetpub\edutech\wp-content\plugins\wordfence\wordfence.php on line 23
[14-Apr-2016 18:38:49] PHP Fatal error: require_once(): Failed opening required ‘lib/wordfenceConstants.php’ (include_path=’.;C:\php\pear’) in E:\Inetpub\edutech\wp-content\plugins\wordfence\wordfence.php on line 23
[14-Apr-2016 18:38:49] PHP Warning: require_once(lib/wordfenceConstants.php): failed to open stream: No such file or directory in E:\Inetpub\edutech\wp-content\plugins\wordfence\wordfence.php on line 23
[14-Apr-2016 18:38:49] PHP Fatal error: require_once(): Failed opening required ‘lib/wordfenceConstants.php’ (include_path=’.;C:\php\pear’) in E:\Inetpub\edutech\wp-content\plugins\wordfence\wordfence.php on line 23

The file wordfenceConstants.php exists in the right folder and has full access permissions.

Things I have tried (with no success):
– disabling all other plugins
– changing themes
– increasing memory limit: define( ‘WP_MEMORY_LIMIT’, ‘256M’ );
– manually deleting the WordFence folder and downloading it directly
– upgrading WordPress (the same things are happening in WP 4.4 and 4.5)

I have multiple other WordPress sites (not networked) successfully using WordFence.

thanks for your help,
Charlie