Unable to upload images

I am suddenly unable to upload any images to the media library. When I try, I get the message “The uploaded file could not be moved to wp-content/uploads.” I have checked permissions on the uploads file in Filezilla, it says permissions are 755. I attempted to change the permission to 777 with Filezilla, and it says, “Operation Denied”. I told my web host, and they said “…it’s a really bad idea to be messing with permissions without any clear idea of what the problem actually is…look at ‘Apache error_log’ in particular and see what’s really going on at the time of upload and form a strategy from there.” I am not seeing any errors in that log that correspond to the time of the attempted uploads. I am seeing some errors that predate the upload attempts by a couple of days. One in particular: “AH01909: RSA certificate configured for (domain_name.com):443 does NOT include an ID which matches the server name.” A couple of other entries in the log mention specific plug-ins, which I have disabled, but I am still unable to upload any media files.

This web host migrated all the WordPress sites to a new Linux server about a month or so ago, and this is the first I’ve tried to upload media files since then, I believe. Never had this problem before.