After you changed the enable-plugin settings in config.ini, you need to run with --replay like follows:
./steemd --shared-file-dir /dev/shm/ --replay
Also you can check if the disk is full:
df -h /dev/shm/
//Update: after further investigation, we found that @felixxx's issue is caused by a typo when running the mount command, so the size of /dev/shm remained unchanged (too small), so unable to allocate new space. Here is the tip: no space after the comma, and, just copy & paste.