Errors when kaltura server under load

256M for a very loaded server can be exhausted. Seeing how you have 8G, I think you can increase it to 512M.
Regardless however, check what I wrote about cache dir permissions and also, try to understand what sort of requests cause the issue. From the error string above, it seems it is related to HTML5lib only? I really do think it is related to a failure in writing the cache.
Also, make sure you have /opt/kaltura/web/html5/html5lib/v2.*/cache/
and that it too, in addition to /opt/kaltura/app/cache, is writeable for the apache user. Note that /opt/kaltura/app/cache should be writeable for both the 'apache' and 'kaltura' users whereas the HTML5 cache is only written to by apache.