Old installation suddenly stopped working

The email settings that have worked for years now, all of a sudden, seem to be doing weird things with Contact Form’s code.
I have two forms: one successfully sends the messages, but when I try to reply, I risk to send mails to myself, as in the “To” field in the tab that opens there’s my own email address and not my site’s visitor’s; so I always have to be careful, remember this, and manually put the visitor’s address in the field. The other form straight-up doesn’t work anymore: a red box appears when clickling “Send”, saying “Failed to send your message. Please try later or contact the administrator by another method.”
Again: I haven’t even entered the plugin’s settings in YEARS. They’ve always worked and I’ve just let them be.
I don’t THINK (?) I can post my website’s link here as it shows NSFW images, but I wait for a clarification on that respect. I could send the link to a private email address, if provided with one.