Well, I figured it out. Basically, any shell command is passed by a security filter. So in practice, texvc is executed by Mediawiki through bin/ulimit4.sh:
#!/bin/bash
ulimit -t $1 -v $2 -f $3
eval "$4"
where $4 is the actual texvc command being run and $2 is the amount of memory allowed for this process. The memory that comes by default is 102400 KB (exactly 100MB), which seems to be insufficient for this process to run. The amount of memory can be set in LocalSettings.php with the variable $wgMaxShellMemory. In my case I set it to 300MB, $wgMaxShellMemory = 307200;, which seems to be enough.
Why this small process of generating a png needs so much memory I do not know.
The reason why this stopped working after updating to Ubuntu 14.04 probably has to do with some newly shipped version of either latex, dvipng, convert, etc. requiring more memory than it did with the version that came with Ubuntu 12.04.
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