As @schoen correctly mentioned above, it is very likely that CSR was generated elsewhere and it was not created correctly.
When CSR is generated by ZeroSSL, such fields as Organization, Organizational Unit and the like (which might have the text you have quoted) are not used. Additionally, if you attempted to enter “comptoir general de robinetterie” as a domain name, you would see an appropriate error popup, indicating than neither of those words is a valid domain name.
The OU and similar fields can be added to a CSR via on-site CSR Generator but that text would trigger the same error message there if entered into wrong input and if entered into the right one, that would not return the originally quoted error message from LE.
P.S. If this is indeed the case of CN set in this way on a pre-generated somewhere CSR, I might add an additional check for that. Currently ZeroSSL actually runs the basic validation against SAN on a provided CSR to ensure that the names are set with type 2 (DNS), so for example if someone puts IPs or emails into SAN, there will be an appropriate error shown. The case of placing some text into CN itself did not seem to have happened before (at least no one raised that as an issue), but I guess it won’t hurt to do some basic check for that too.