I have the same problem here. The combination of accents/umlauts and commas is encoded wrongly in SOGo.

a)
The display name "Max Müller" must be (and is)
from: =?utf-8?q?Max_M=C3=BCller?= <emailaddress>

b)
The display name "Mueller, Max" must be (and is)
from: "Mueller, Max" <emailaddress>

c)
The display name "Müller, Max" must be (and is NOT)
From: =?ISO-8859-15?Q?=22M=FCller=2C_Max=22?= <emailaddress>
(copied from thunderbird)
As you can see, the comma and the quotation marks are encoded too.

Instead SOGo does it the wrong way
from: =?utf-8?q?M=C3=BCller,_Max?= <emailaddress>

This is the same bug as 0002120.

Please try the upcoming nightly builds.