BuddyPress.org

Uploading big files on your site by you or site users, like video files, need much memory, because a part of the file is stored in your server memory. Big files, much more memory.
Well, i think that 128M RAM is ok if you will not upload big files.( more than 100M).
Also, Wp Super Cache need much memory than other scripts…
If you will get that memory error again, change the memory limit. (256 if possible)