@pdehombreux Sorry for leaving you alone for such a long time on your very first post. It’s been a very busy time and I just couldn’t get my head around your post when I first read it and so kept on putting off to look into this.
I think we need to get into the logs to see what’s actually happening when you try to update the AD information. First check the apache error and php-fpm logs right after (or while) you update the client’s AD information. See my signature on where to find those.
If that does not shed a light (I suppose it won’t) then I may ask you to enable mysql query logs. For that it’s probably best to turn off as many components as possible first. Shut down clients that have the FOG client installed and make sure you are the only person on the FOG web UI for that moment. Just trying to keep the logs to a minimum.
See here on how to enable mysql query logging: https://www.pontikis.net/blog/how-and-when-to-enable-mysql-logs
Before you enable it, get the web interface ready. Navigate to the right host, select AD settings and change OU and username for example. Then turn on logging, restart mysql, update the client settings and switch off mysql logging again. Upload the full log here (make sure it does not contain any sensitive data …)