(Phil and I typing at same time... but since I already typed it in...)
Not enough info to do any debugging... no link to gallery... no specifics of what the folder names are (special chars were one issue)... and likely admin access would be needed to view (do not post admin credentials here).
The CHANGELOG.txt file in the latest download will like ALL the changes... a simple find of 'batch' shows the fixes - and the related threads that detail the issues - if you want to chase down that path.
The most relevant appear to be:
2015-05-30 Fixed batch-add interface if directory name contains specific strings (thread ID 78202) {eenemeenemuu}
2014-10-06 Fixed several issues with file path names on batch-add (thread ID 77820 & 77823) {eenemeenemuu}
2014-07-01 Fixed batch add interface issue when selecting folder (thread ID 77442) {eenemeenemuu}
2013-05-02 Merged missing code changes of error message support for the batch add process {eenemeenemuu}
2011-07-27 Fixed batch-add issue if no file is selected (thread ID 70342) {eenemeenemuu}
Many issues have been due to new versions of PHP being implemented on webhosts - introducing changes in behavior... If you have the ability - try pointing your site to an older version of PHP (talk to your webhost about if you can do this or how to do so - may be via control panel settings, php.ini, or .htaccess files depending on the host.) See if they documented any upgrades.
With a 6 year old version/release - there are MANY fixes including security patches that should be applied. If you don't have any manual code changes to CPG, the upgrade should go very easily...
Quite honestly we are a small group actively maintaining CPG - and debugging 6 year old code isn't going to be a priority... Keeping the code current and working on the next release is where we spent the precious time we have.