As a lot of invitations are sent via SoSci Survey, some email provider rank www.soscisurvey.de higher, regarding the SPAM classification. So, the emails sent via SoSci Survey may have been filtered.
Should you encounter this issue, either tell the respondents to check the spam folder (and tell their email provider or email programm that this is no spam) or tell SoSci Survey to send the emails through an SMPT server of your own (Use SMTP server). For security reasons, it is usually useful to create a dedicted account/postbox on your SMTP server before using that.