not working

Hi

I have been using this plugin since long, but suddenly I don’t know how and why this plugin is working strangely.

When I download files using this plugin it does not complete it. Instead it shows “Failed”. Tried various browsers and systems. Although sometimes (once in blue moon) it works fine.

All the testing I did:

• Deactivated all the plugins, but no use
• Changed theme to default twentysixteen but again no use
• server logs-
  [22-Jul-2016 07:27:51 UTC] PHP Notice: Undefined variable: regparams in www/wp-content/plugins/download-manager/tpls/wpdm-reg-form.php on line 19
  [22-Jul-2016 07:27:58 UTC] PHP Notice: ob_flush(): failed to flush buffer. No buffer to flush in www/wp-content/plugins/download-manager/libs/class.FileSystem.php on line 128
• Tried to put same files on root folder for download to check if there is any problem with the web server, but it worked fine too.

Do you have any idea what can be the problem?

Thanks

https://wordpress.org/plugins/download-manager/