Comment (by Federico Cipolletta):

Apologize me for my error in using the terminology “backward compatible”. The problem is that the GRMHD code I was compiling is working correctly with FFTW 3.3.7 (with which it was developed), but it does not seem to work with 3.3.4.

I did not try to build the library from scratch because I already had it installed.

I already reached a solution, by installing FFTW 3.3.7 on another location (/home/federico instead of /usr/local) but it took some time for me to figure out what was going on. My proposal for adding one comment was meant for un-experienced people (like me ^_^).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2428/possible-issue-between-externallibraries