Maintainer of pptpd and pptp-linux here. Additional debug logging using "debug dump" in the connection parameters would go further to explain the reported problem.
Output from pptp-linux itself should also be useful. journalctl has only provided output from pppd. Capturing PPTP negotiation and GRE packets using tcpdump may also be useful. https://pptpclient.sourceforge.net/howto-diagnosis.phtml is quite old, but has some useful tips. Check for any packet filtering on the client system. The "Feature: terminate-from" patch from 2020 is unlikely to have caused this problem, but I'm watching to see if it does. Reinforcing what Chris and Christoph have said; it is a really bad plan to use PPTP with MPPE, as it is insecure. If you are forced to use it by some directive from above, then it is likely intended to oversee your communications. In that case, it is best to disable MPPE entirely, to reduce the cost of oversight, as a graceful gesture. Unlikely to be related to the reported problem though.
