I have been running roundcubemail 0.5.1 on an Apache server with eAccelerator 0.9.6.1 for some time without any issues.
After upgrading to 0.7 my users complained they could no longer view attachments.
On closer examination I saw Apache was segfaulting
The workaround I found was to disable eAccelerator.
Since there were other applications on the server that benefited greatly from eAccelerator I selectively disabled it for roundcubemail scripts using the
eaccelerator.filter option.
It would be nice to understand why roundcubemail suddenly has a problem with eAccelerator
Its a known problem:
#1488262 (httpd crashes with segfault when attempting to view attachments) (http://trac.roundcube.net/ticket/1488262)
http://trac.roundcube.net/ticket/1488256
This does not answer your question directly, but you could switch to APC, it does a nice job on PHP Opcode caching ans plays nice with Roundcube.