Hello Roger,
My problem had been self-inflicted. I had moved a site from one Apache server to other servers: one running on WAMP and one on Linux. I was trying to solve an issue of certain images not loading on the Linux server. Having solved loading issues previously with SVG files, I started by adding mimes to my .htaccess file
AddType image/gif
AddType image/jpeg jpeg jpg
AddType image/png
It didn’t fix the problem, but caused the issue with the CAPTCHA.
I finally found the cause of my image loading problem, which was that some images had some upper-case letters in the file name. I had forgotten that servers running on Windows can accept upper case, thus no problem on the WAMP server. However, those on Linux generally do not, although the previous server ran on Linux and the images had been displaying with no problems.
I still don’t understand why adding these mimes, actually caused the CAPTHCHA problem and would appreciate further comments about this issue.