PLEASE NOTE, THIS IS NOT A BUG WITH A PLUGIN, THIS IS A BUG WITHIN DOKUWIKI'S TARLIB CLASS BEING UTILIZED BY A PLUGIN.
Creating a valid tar file fails when a file name plus path is greater than 100 characters in length.
Once this happens, subsequent files added to the archive tend to not be available in the output tar file.
Because of Dokuwiki required paths, this limits page names plus name spaces to roughly 75 characters.
Attic files are limited to even smaller file name plus name space lengths.
For example, a page with the name:
(We shall call this a "trigger page name.")
Will fail to be added to the archive properly, and subsequent files will likely not be added properly either.
TO TEST THIS BUG:
1. Please test with an active wiki with a good amount of pages and attic files
2. Create a page with a "trigger page name." (see above.)
3. Download and install Andreas Wagner's version of the BackupTool available at 
4. Create an archive of pages and revisions (which are attic files)
5. Download the archive, ungzip and untar, make any note of untar errors.
6. Compare the "pages" and "attic" directories within the untarred archive to the original file system. Make notes of any discrepancies.
Your goal at this point would be to compare this against the tar file format specification, or perhaps the source code to GNU tar. Fixing these discrepancies should fix the problem altogether.
To aide you, find attached email conversations I had with the Maxg TarLib developer back in September 2006 (this is the same bug.) Please note that I have verified that the archive program 7-zip is not at fault since these problems also occur with GNU tar, BSD tar, and Mac OS X Archive Utility.