The last few days my host's firewall has activated and blocked my IP. After quite a lot of testing of other possibilities (hardware and software) I believe I have narrowed it down to an issue with Wordfence. I have it installed on a number of sites on the same host/server and it is behaving the same way for all of them.
I haven't tried to block my own IP so I don't believe it is caused by my blocking my own IP! Also I have NOT password protected the wp-admin folder and admin-ajax has normal file permissions.
Admin-ajax also works fine when called by other plugins.
And I believe that the only rewrite rule I have to do with wp-admin is as follows:
RewriteRule ^wp-admin/includes/ - [F]
I don't think it is a corrupted version of Wordfence - since it is happening on all sites I am using it on.
The hosting company doesn't have its full complement of staff in today (Saturday) - but they have emailed me the two files that are allegedly causing the problem:
I have checked both of these files and neither seem to be any different from the files that are in the version of Wordfence I just downloaded to compare. Perhaps they are just a red herring.
I do have another security plugin installed (iThemes Security). I have tried disabling it to see if it stops the problem but both the ajax 404 message and eventually the firewall issue with the host are still happening.
Would appreciate any advice. Thanks.