Destination folders of all filters get lost from time to time. I do not know what is causing that loss of the filter settings but it happens every few week. All filters (spam, ML, virus detection, etc.) suddenly stop working. When looking into the filters, all destination paths are empty. When re-selecting all filter's paths everything works as before. The rest of the filter settings seem not to be influenced by this. Only the path get lost. Reproducible: Sometimes This is about KMail 4.14.6 (is not in the list above)
This bug has never been confirmed for a Kontact version that is based on KDE Frameworks, except possibly a Technology Preview version 5.0.x. Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the opportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.