Subscribbing seems to have been broken

I have a client that is using Subscribe2 and after updating WP core and other plugins recently, the the whole subscribe system seems to have been broken. Any attempt at subscribing simply gives this error:

Sorry, but that does not look like an email address to me.

A single attempt to enter an email throws dozens of errors:

[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant IDNA_NONTRANSITIONAL_TO_ASCII – assumed ‘IDNA_NONTRANSITIONAL_TO_ASCII’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: Use of undefined constant INTL_IDNA_VARIANT_UTS46 – assumed ‘INTL_IDNA_VARIANT_UTS46’ (this will throw an Error in a future version of PHP) in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: idn_to_ascii() expects parameter 2 to be integer, string given in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1037
[05-Mar-2020 21:56:36 UTC] PHP Warning: checkdnsrr(): Host cannot be empty in /home/soullici/public_html/wp-content/plugins/subscribe2/classes/class-s2-core.php on line 1041

WP and S2 are up to date as of today. I already tried deactivating all plugins except S2. I also tried a few themes, all the same situation.