Several security plugin options, which increase security to the blog, are causing problems with new requests (changing the month etc.) and are displaying 403 errors.
More information on this support item: https://wordpress.org/support/topic/403-error-or-less-security?replies=7
A solution was offered > disable the several security options.
I understand that this is a 'Solution', but these are only temporary solutions.
I do hope a permanent fix for this problem will be found.
As this is just a minor settings within the security plug ins, for this time I'll disable it.
But these security settings are there to increase the security of the websites and I think most of us will agree that you want to keep your websites as secure as possible. I would consider finding alternate plug-ins, to keep the security settings in place. And I'm taking this into consideration for future projects, I like this event plugin, but security is a hot issue, with some first hand experience in hacked sites, I give this a higher priority at the moment.
As said, I hope a more perminent fix can be found for this problem. And via the forums, the suggestion to put it here was made.