May be this will be useful for some users:
I've installed blog and ecommerce module, also i've created WhateverPage.php in mysite/code. After running the db/build (SS.2.2.3) and working all day (during the day i have deleted the WhateverPage.php and run db/build. The next day I've come and see that admin page is not longer working and got 500 error.
So, i've enter in dev mode put in yourproject/_config.php Director::set_environment_type('dev');
I saw that WhateverPage class was missing. Looked in dbase into the Site Tree and it was good, without WhateverPage. Was a cache problem.
Scond step put in yourproject/_config.php HTTP::set_cache_age(0);
Clear browser cache, and runf flush, also you could run db/build one more time.