All files users attach to topics or replies are stored in media library. But, before that, they are all checked to determine if they are allowed (size, format). During upload various problems can occur to break upload or corrupt the files. All these problems and errors will result in file not being uploaded, but instead an error entry is generated and saved and later displayed with the topic or reply, or in a log.
All errors can be seen on the Errors Log panel in the Toolbox admin menu. This panel grid looks similar to Attachments panel, but it shows errors for every upload problem. This can help you to debug problems with upload process. But, with GD bbPress Toolbox Pro 3.3 and newer attachments process is improved to show validation before upload, so that oversized files or files with wrong file type are prevented from upload, and this will result in less errors during upload, and less errors displayed in the errors log.
To configure logging process, you need to open plugin Settings -> Attachments panel. There you have a options group ‘Errors Logging’. This has several options. First option is used to activate logging process. If this is disabled, no errors will be logged! After that you have options to control who can see the errors. This is related to displaying errors in the topic or reply to inform about the errors on the front end. Normal users or visitors can’t see these logs, they are reserved for administrators, moderators and topic/reply author, and in the settings group you can enable or disable this.