Please check there are no template overrides in the template folders html folder. If you done a template override on the old gallery version, there will be some files in your templates html/com_igallery folder. These files will override the gallery output and they will output html that might not work with the new gallery version. If you have not done any template overrides, please ignore this point.
Please also check if a joomla caching plugin is turned on. If it is, please check the cache is cleared.
If this does not fix the issue, please contact ignite gallery support