Fatal Error Upon Plugin Installation and Activation

I have had the CleanTalk plugin for a while but recently noticed there were PHP fatal errors being logged sporadically that traced back to CleanTalk. The error was as follows:

PHP Fatal error:  Uncaught Error: Undefined constant "Cleantalk\ApbctWP\CURLOPT_USERAGENT" in /wp-content/plugins/cleantalk-spam-protect/lib/Cleantalk/ApbctWP/Helper.php:42
Stack trace:
#0 /wp-content/plugins/cleantalk-spam-protect/lib/Cleantalk/ApbctWP/Helper.php(138): Cleantalk\ApbctWP\Helper::httpRequest()
#1 /wp-content/plugins/cleantalk-spam-protect/cleantalk.php(876): Cleantalk\ApbctWP\Helper::httpRequestRcToHostTest()
#2 [internal function]: apbct_sfw_update__init()
#3 /wp-content/plugins/cleantalk-spam-protect/lib/Cleantalk/Common/Cron.php(265): call_user_func_array()
#4 /wp-content/plugins/cleantalk-spam-protect/cleantalk.php(192): Cleantalk\Common\Cron->runTasks()
#5 /wp-includes/class-wp-hook.php(307): {closure}()
#6 /wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters()
#7 /wp-includes/plugin.php(474): WP_Hook->do_action()
#8 /wp-settings.php(587): do_action()
#9 /wp-config.php(89): require_once('...')
#10 /wp-load.php(50): require_once('...')
#11 /wp-blog-header.php(13): require_once('...')
#12 /index.php(17): require('...')
#13 {main}
  thrown in /wp-content/plugins/cleantalk-spam-protect/lib/Cleantalk/ApbctWP/Helper.php on line 42

I tried deleting the plugin and installing a fresh version from the plugin page and was met with the same error when trying to activate it. Please advise, thanks!