Thin start --ssl is aborted with "ee key too small" error in Ubuntu 20.04

[Rael G.C.]

I just installed Ubuntu 20.04 (which is based on Debian 11), with ruby 2.6.6 and thin gem 1.7.2.

When I start thin (`thin start --ssl`) it starts fine, but when I try to load any page in the browser, thin is aborted with the following error:

139731575054016:error:140AB18F:SSL routines:SSL_CTX_use_certificate:ee key too small:../ssl/ssl_rsa.c:310:
ruby /home/rael/.rvm/gems/ruby-2.6.6@output-set/bin/thin start --ssl --trace --debug: ssl.cpp:203: SslContext_t::SslContext_t(bool, const string&, const string&, const string&, const string&, const string&, int): Assertion `e > 0' failed.
Aborted (core dumped)

I'm assuming this is a problem with the cert file using weak encryption. But which is the cert file used by default, so I can try to re-generate it?