@egabillat, @fruitful:
Perhaps I can help. (I’m the developer of WP-SpamShield) This has been listed on our known issues page for some time: https://www.redsandmarketing.com/plugins/wp-spamshield/known-conflicts/
Maintenance alters the WordPress login process. Hooks from the standard WordPress login page process are missing in the Maintenance plugin. (Not saying this is bad, just explaining.) WP-SpamShield depends on these to check if it’s on a login/registration page, add code to the login page in order to validate, etc. This would affect compatibility of many plugins, not just WP-SpamShield. Keep in mind that WordPress login & registration process are somewhat tied together so some of the hooks affect each other.
@egabillat:
As of right now, to use these plugins together, be sure that you check the option for “Disable anti-spam for miscellaneous forms” in the WP-SpamShield settings.
@frutiful:
If you make sure it has all of the standard hooks that fire throughout the login process, it will help fix this issue. (For example, WP-SpamShield looks for ‘login_init’, ‘login_head’ hooks to detect login pages with non-standard names, to make sure it doesn’t run the Miscellaneous Form Anti-Spam check on login pages.) Otherwise the plugins will remain in conflict. If you would like to contact us to work on bridging compatibility gaps, we’ll be happy to help.