Error 500 by activating the pressbook-plugin


I just installed the latest version of the pressbook. When activating the plugin (2.6. from the installation-instructions here:, I receive a 500 error.

My log says:

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to undefined function HM\\Autoloader\\register_class_path() in [MYPATH]/wp-content/plugins/pressbooks/compatibility.php:135\nStack trace:
\n#0 [MYPATH]/wp-content/plugins/pressbooks/compatibility.php(70): pb_init_autoloader()
\n#1 [MYPATH]/wp-content/plugins/pressbooks/pressbooks.php(121): pb_meets_minimum_requirements()
\n#2 [MYPATH]/wp-settings.php(268): include_once('[MYPATH]...')
\n#3 [MYPATH]/wp-config.php(95): require_once('[MYPATH]...')
\n#4 [MYPATH]/wp-load.php(37): require_once('[MYPATH]...')
\n#5 [MYPATH]/wp-admin/admin.php(31): require_once('[MYPATH]...')
\n#6 [MYPATH]/wp-admin/network/admin.php(13): require_once('[MYPATH]...')
\n#7 [MYPATH]...
\n', referer: [MYURL]/wp-admin/network/plugins.php

Any Ideas?


Looks like you didn’t follow the second step here. The auto loader file needs to be in mu-plugins.

Hi Ned, thanks for your answer. Yes, I did, the file “hm-autoloader.php” is located in the directory “/wp-content/plugins/mu-plugins”. The error persists.

Hi again, I was wrong, I had copied the autoloader in

“/ wp-content / plugins / mu-plugins”

and not in

“wp-content / mu-plugin”.

I corrected it and now it works. Thank you!

But it is supposed to be in wp-content/mu-plugins, not wp-content/plugins/mu-plugins. Please review the instructions.

yes, that was the mistake, now work it. Thank you very much!