I will nevertheless share three links which were particularly edifying:
1. A concise definition of the that-trace effect from the Lexicon of Linguistics: https://lexicon.hum.uu.nl/?lem...ct&lemmacode=148
2. A more detailed overview of the effect (here, listed under its original name, the Comp-trace effect): https://www.ling.upenn.edu/~be...ch11.html#comp-trace
3. And, finally, a paper published by linguists at the University of Zürich discussing the that-trace effect in English, German and Swiss German: https://www.zora.uzh.ch/id/epr..._can_be_repaired.pdf
Thank you very much for sharing that material. Your gesture is highly appreciated.