Some of this post (the cksum bit) assumes that you're running on Linux, rather than Windows, but the basic idea should be the same either way; if you are on Windows, just skip the cksum bit.
I think the getMenuIcon function should be defined in <DokuWiki root>/lib/plugins/admin.php
It's very small:
public function getMenuIcon() {
$plugin = $this->getPluginName();
return DOKU_PLUGIN . $plugin . '/admin.svg';
}
Do you see that there? Could you provide the output of this command, replacing <DokuWiki root> with your actual path:
cksum <DokuWiki root>/lib/plugins/admin.php
I'm also running the latest F-Manners, and for reference, mine is:
$ cksum admin.php
2218900782 2571 admin.php
Alternatively, how many lines long is that file? I think it should be 96 lines long at this point.
Also, are the ownership and permission of this file the same as all other files around it, or are they different? How did you perform your F-Manners upgrade? Was it manually, or using the Upgrade plugin?
Also, when you say you "dropped back a version", how did you achieve that? Did you revert the entire [virtual] machine, or did you somehow just roll back files?
In short, I suspect you have an old version of <DokuWiki root>/lib/plugins/admin.php, because
the function which is missing in your error message was only added this year.
If you're lucky, it's just this one file, but you might have wider installation issues.