Try deactivating the Enhanced uploader in your general settings

Click on the Settings link at the top. In the settings, click on General Settings (on the left). Scroll down and find the area that says:

Enhanced Attachment Uploading:
Enhanced Attachment Uploading on
Enhanced Attachment Uploading off


The forum features an enhanced attachment uploading system that allows you to reuse existing attachments via a drag and drop interface.
Depending upon the capabilities of your web browser, you may not be able to use the enhanced system. If you experience problems with the interface, try disabling this option.

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As it says, please try disabling this option and see if that helps.

Thanks!