libx264 aborts with unable to parse option value "0.6"

My setup: kdenlive 0.9.3+git from sunab's svn repository+mlt+libav
Kubuntu 12.04 64bit

Rendering of a clip is aborted using the H264 codec:

[libx264 @ 0x7f49589ae9e0] [Eval @ 0x7f495dffb490] Invalid chars '.6' at the end of expression '0.6' [libx264 @ 0x7f49589ae9e0] Unable to parse option value "0.6"

Happens only a single project that I recently changed from 720p/25Hz to 1080p/25Hz. Checking the transitions I could not find any problem indication.

Any suggestions?

Forums:

Trimmed down the project file until I found the problem: somehow kdenlive generates some invalid parameters when handing over the job to rendering to mlt. I could track down the problem to a particular effect on a certain clip. Disabling and then reenabling seemed to have caused parameter regeneration and since then my project can be rendered without problems again. Strange...