If you are getting a thread error like "libgomp: Thread creation failed: Resource temporarily unavailable" when you execute imageMagick commands, then you will have to compile ImageMagick with the param --without-threads
on the configure command. The first thing you have to do is remove the imagick package for php (php53-imagick) and then procede to the compilation:
$ ./configure --without-threads ... $ make && make install
Now you can reinstall the package php53-imagick, so you can use it in your PHP applications.