This error occurred on a friend's computer after the first installation on Windows 7. It seems not a memory issue as long as the consumption were alright. The only things done were opening the client, logging in and creating a char.
1. At first need to be sure what all updates downloaded.
2. If this issue still present, this is bug in OpenGL driver, and need disable one option in ManaPlus, in settings (f9), performance tab, disable option "enable texture atlases"