Thanks for the details, @Olivier_a and for the troubleshooting, @danbjoseph. @Olivier_a what are your server specs? Do you have more than 1GB of RAM? If not, have you added swap?
If you log into your server backend and type htop (or top if you don't have htop), then try to access an Enketo form, does either your RAM or CPU max out?
Related: Unable to test a form on odk central