All old flight plans (*.flt) disappeared - after FS patch 1.15.10.0?

Do you have any add-ons in your Community folder? If yes, please remove and retest before posting. no addons than the default MSFS World Updates

Are you using Developer Mode or made changes in it? no

Brief description of the issue:
When I saved a new flight plan today, if was presented with an empty (or new?) directory without any *.flt or .pln files. ( somewhere in …\AppData…\LocalState\ ). Thats ok.
But: All old flight plan files (
.flt and *.pln) disappeared and cannot longer be found on the local drive C: (location of user directory and of MSFS installation). Only the default MSFS flightplan files are found
Latest change was the new patch 1.15.10.0.
BTW: What was the old location of saved flight plans?

Detail steps to reproduce the issue encountered: unknown cause. Maybe the patch 1.15.10.0 installation

PC specs for those who want to assist (if not entered in your profile): Windows 10

Build Version # when you first started experiencing this issue: After patch 1.15.10.0

Are you on the Steam or Microsoft Store version? MS Store (from DVD) / Deluxe package

Did you submit this to Zendesk? If so, what is your ticket #? Request #104696