Enabling Order and Order UI crashes site

It sounds like you may be running up against resource limitations in your development environment. Why don't you try installing via Commerce Kickstart to ensure there's no funny business going on; it does a full installation of Drupal Commerce during the standard Drupal installation process, preventing these types of problems.

http://drupal.org/project/commerce_kickstart

Forgot to tweak the status. Feel free to follow-up if you can't get Kickstart to work for some reason, but 9 times out of 10 it's a memory limit thing.

Hello,
I'm having the same problem and don't know how to fix it based on the responses given.
Attempting to enable commerce modules has crashed my site, which unfortunately is live and not a development site, so any prompt advice would be appreciated.

I was attempting to add commerce functionality to my existing site, so I did not use commerce_kickstart (which seems to assume you are starting from scratch?) and I'm not sure now how to backout and disable the modules because I cannot access my admin pages nor run any drush commands.
Loading any pages on the site gives
The website encountered an unexpected error. Please try again later.
and running ANY drush command gives the same
entity_views_field_definition
error message as above.

Also, I'm not sure that it's a memory limit problem, because I believe I had upped it many weeks ago.

$ grep memory_limit /etc/php5/apache2/php.ini
memory_limit = -1      ;

$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 202752
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 202752
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

Again, any help would be much appreciated.

There's nothing inherent in Drupal Commerce that will cause a site to crash on a simple installation. Double check your resources and error log messages to confirm your scenario. Additionally, please don't test Drupal Commerce on a production site; the absolute best thing you can do for yourself is to install Commerce Kickstart locally and implementing your requirements before trying to add it to a production site. Then you can duplicate the site locally and add the Commerce functionality on there using your Commerce Kickstart installation as a guide.

You can disable modules directly in the database in the system table. You should find information about that on a search of d.o or Google.

Added log message