I have spent quite a bit of time analyzing this error using a test gallery of some 500 image files added via the file system and make the following comments:
_metadata.resources is an xml file located in the top level gallery and each child folder of a gallery. Whenever a page containing the gallery module is visited following a period of inactivity, the elements of this xml file must be reread to build a collection of folder and media file items. This collection is then cached to make subsequent visits to the gallery load more quickly. Each time a new child album or media file (via the module interface or via FTP) is added to a gallery album or any changes are made to a media file's metadata, this xml file must be rewritten. The more media files already contained in the gallery or the more being added, the longer the process of rewriting the xml file will take. It is during that time should another user visit the gallery triggering the rewriting process that this error will be thrown.
Once the xml file is completely written, any subsequent visits to the gallery, even if the cached data has expired, will require only reading, not writing the xml file. Voting will, however, require a rewrite and probably should be disabled for a large gallery.
Should the initial rewrite of the xml file following the addition of media files be interrupted by this error being thrown when another user visits the gallery or a user becomes impatient and clicks on another page or album link, the _metadata.resources file may never be completely built thus triggering a rewrite and more of these errors each visit to the gallery. This is what I suspect is happening in your case, even though you are not adding any more files or making edits to the gallery.
If you could take a look at the _metadata.resources files in the top level gallery and each child album and advise if a) there is a <file> node for each and every media file in the gallery; b) if the <id> node of each <file> node contains an integer and c) the <approveddate> node of each <file> node contains a date either earlier than the current date or the minimum date marker which begins with 9999-12-31, it would be helpful. If you would prefer, you could email the _metadata.resources files to me at bill(at)wesnetdesigns.com.
This form of data caching is not at all related to the module's cache time which controls output caching and which (as you found out) cannot be used for any interactive module. It is not possible nor desirable to disable the data caching of the gallery metadata.
You might also try to temporarily set the view permissions of the page containing the gallery to exclude all users except yourself, click on the refresh icon in the top or bottom left of the gallery, and give the gallery sufficient time to complete rebuilding the _metadata.resources files, then reset the page view permissions to open the gallery to all users. Once the xml files have finished rebuilding and provided you make no changes or additions, this error should not continue to occur.