This issue is caused by mods which have not even been updated to 1.36.0 or by a corrupted installation which for some reason has an older version of the file.
Maybe there is a mod active which you are not aware of(the launcher activates mods automatically in some circumstances). And have you already tried to
verify the integrity of game files? If that doesn't help, you can try a
clean reinstall as described in
my guide on reddit about common startup problems with version 1.29
No, it didn't. Plugins which had an outdated version of the file
interface/countrymissionsview.gui
have been missing the buttons since 1.36.0 has been released. The OP is using the mod
banners 5.0 and if you look at the comments for the mod in the steam workshop, you can see that people have complained about the missing buttons since shortly after the 1.36.0 release.
Or do you mean that the plugins which have been updated to 1.36.0 have been working till last night? That's of course correct, because they were not outdated yesterday. But they are outdated now and need to be updated so that they can incorporate the changes to the buttons which have been done in 1.36.1.