Sorry for the inconvenience @koepnick - while searching across all repos has required being logged in for a long time, when we enhanced the search capabilities earlier in the 2023 we had to extend this to repos as well (see https://github.blog/changelog/2023-06-07-code-search-now-requires-login/).
This is primarily to ensure we can support the load for developers on GitHub and help protect the servers from being overwhelmed by anonymous requests from bots etc.
Anyone without access to their old email also loses their account. I don't remember which email address I used with my account back in the day (it's at least ten years old), and since I bought my key from a reseller, I don't have a receipt. Microsofts response was basically "not our problem, guess you'll have to pay us again ¯\_(ツ)_/¯"