If it is only a single person who encountered the problem, there may be very different reasons, and you may not be able to solve it at all...
- It might be a temporary problem with the SoSci Survey server - this is quite rare, any usually occurs only for a minute or so, but it happens. There is also a scheduled maintenance about 3:00 am (CET) where the server may reboot.
- It may be a temporary network problem - the Internet is quite a complex system, and it occures, that some part of the network may be separated from another part for a while. Again, this is a rare occasion, but happens.
- Then it may be a temporary problem with the Internet connection of the respondent (e.g., a WiFi connection),
- or a problem with some software component (e.g., a VPN tunnel) on the respondent's computer that temporarily fails.
And finally, it could actually be an issue with your questionnaire. But in the by far most cases, a programming issue will cause an error message. If the questionnaire simply does not continue (and this does not affect all respondents seeing a speicifc page or stimulus) then it it likely another issue.