Mail sending broken

After an update to WP version 4.3 the PDB doesn’t send emails anymore.

Following Error was in my PHP Errorlog:
PDb_Signup::_mail sending failed for: [mail_address]

[pdb_signup] is on a single registration page.

Tested PHP versions: 5.3, 5.4 and 5.5

Thanks in advance.

https://ww.wp.xz.cn/plugins/participants-database/