Internet-delivered psychological treatments (IDPT) are seen as an effective and scalable pathway to improving the accessibility of mental healthcare. Within this context, treatment adherence is an especially relevant challenge to address due to the reduced interaction between healthcare professionals and patients, compared to more traditional interventions. In parallel, there are increasing regulations when using peoples' personal data, especially in the digital sphere. In such regulations, data minimization is often a core tenant such as within the General Data Protection Regulation (GDPR). Consequently, this work proposes a deep-learning approach to perform automatic adherence forecasting, while only relying on minimally sensitive login/logout data. This approach was tested on a dataset containing 342 patients undergoing guided internet-delivered cognitive behavioral therapy (G-ICBT) treatment. The proposed Self-Attention Network achieved over 70% average balanced accuracy, when only 1/3 of the treatment duration had elapsed. As such, this study demonstrates that automatic adherence forecasting for G-ICBT, is achievable using only minimally sensitive data, thus facilitating the implementation of such tools within real-world IDPT platforms.