I reproduced this with the following list of users pasted into the text box:
Mária Dobøemyslová
Veronika Rudolfová
Marc22
RoreiD
Petra Skalníková
Kateøina Kuèerová
Malástudentka
V-andrea
Daniel Hašek
Tomáš Mihulka
Botanick
VerCapkova
Janneda
Omphalotusia
The resulting cohort showed the following invalid users:
Tomáš Mihulka
Mária Dobøemyslová
Malástudentka
Kateøina Kuèerová
Daniel Hašek
Tomáš Mihulka and Daniel Hašek display as "Tomá Mihulka" and "Daniel Haek" in the username column (https://metrics.wmflabs.org/cohorts/4788/membership) but copy/paste as above (the usernames of all invalid users listed above were c/ped from the username column on the cohort page, not from the original list).
Firefox 36.0, Ubuntu 14.10.