Sorry, going to be an Internet lawyer for a second.
ONLYOFFICE clearly didn't read the AGPLv3. They are saying Euro-Office is in violation of section 7. A logo isn't a legal notice so wouldn't fall under 7b and requiring the logo to remain is a "further restriction" that isn't covered by 7b so that restriction can be removed.
https://github.com/ONLYOFFICE
Relevant parts of AGPLv3
"""
7. Additional Terms
...
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it; or
...
All other non-permissive additional terms are considered "further
restrictions" within the meaning of section 10. If the Program as you
received it, or any part of it, contains a notice stating that it is
governed by this License along with a term that is a further
restriction, you may remove that term.
"""
ONLYOFFICE additional terms
"""
Pursuant to Section 7(b) of the License you must retain the original
Product logo when distributing the program. Pursuant to Section 7(e) we
decline to grant you any rights under trademark law for use of our
trademarks.
"""
#GPL #ONLYOFFICE #eurooffice #nextcloud