Maybe caused by using <? instead of <?php

I think the problem in your php config

But <? is not default PHP config value. It is optional. But you should not depend on optional values.

See PHP documentation: https://secure.php.net/manual/en/language.basic-syntax.phptags.php

PHP also allows for short open tag <? (which is discouraged since it is only available if enabled using the short_open_tag php.ini configuration file directive, or if PHP was configured with the --enable-short-tags option).

Your are the first, whom I meet, who don't allow short open tag by default... But fixed it

C'mon, the short open tag is deprecated and it does not default. You should be less arrogant when somebody tests your plugin for free.