I am using Version 3.15.9+190214
You shouldn't.
This "thing" is from February 2019, so almost 4 years old now. That means no security updates, no bug fixes for almost 4 years. In times of GDPR (or for you DSVGO) collecting data with a software that hasn't been updated for almost 4 years is really quite "fahrlässig", I'd say.
Should anything happen in terms of a data breach, you really can't say that you did everything to protect the data, with a software that hasn't been updated for so long. Limesurvey is free, there are no license fees involved, so an update doesn't cost anything (only if you use ComfortUpdate - which is NOT necessary - though I recommend it because it makes the update more comfortable, it literally takes less than 5 minutes).