I just found the same issue posted in the pre-membership forum – to answer the questions asked there..

Language.. no language site is in English..

No plugins running that use any language.. I also deactivated them all

I am on WP 4.0

But SP is now on version 5.1.1…

This error is in the WordPress code so it is hard to know what the cause might be. Before anything else I would advise on SP 5.1.1 which is needed for  WordPress 4.0 – AND update any SP plugins you have active as well.

Then see what happens…

The error is only affecting the SP forum page.. all other pages and posts do not have this issue – so I would assume it related to SP

I just googled the error / warning and the search brings up a lot of sites with this same warning.. all/most running SP

https://www.google.com/?gws_rd=ssl#q=Warning%3A+in_array()+%5Bfunction.in-array%5D%3A+Wrong+datatype+for+second+argument+in+%2Fwp-includes%2Fformatting.php+on+line+324

If you have upgraded WordPress to version 4 as you say then it IS required to update Simple:Press to version 5.1.1. There are dependencies. So after you upgrade if you still get the error then we may have something we can look into but that step is necessary.

So are you saying 4.5.1 will not work with WP 4.0?

Also can I upgrade directly from 4.5.1 to 5.1.1 following the upgrade instructions?

support for the 4.x series ended a long time ago (2 years?)… the last supported wp version of 4.5.x I believe was wp 3.2… so we cannot really asses how well it would work…  lots of things were broken since then from compatibility standpoint..  it may have continued functioning but some stuff would not work…  we only bump the compatibility number when wp forces a change upon us…

the required wp version for 5.5.1 is wp 4.0…

for upgrading from 4.5.1, please see:  https://simple-press.com/documentation/codex/installation/upgrading/upgrading-version-4-5/

So are you saying 4.5.1 will not work with WP 4.0?

Yes or No?

Also can I upgrade directly from 4.5.1 to 5.1.1 following the upgrade instructions?

Yes or No?

sorry, not sure how to be more clear…

Since 4.5.1 is past end of life, we have never tested it against wp 4.0 (last version we tested against was like 3.2)…  so cannot give definitive answer, but I would expect it would not work with wp 4.0…

I gave you a link with the exact steps to upgrade 4.5.1 to 5.1.1…  so yes, follow those instructions…

Since published instructions are not always up to date I was simply attempting to confirm that 4.5.1 can be upgraded to the current version as published.. without the need to go to a middle version as part of the process..

in this case the published instructions are up to date but as you are using version 4.5.1 then there is no intermediate version you need to upgrade to first.

got it.. the warnings  / error messages are gone after updating to 5.1.1 –

sounds good… thanks for the update!