Same issue here. I have LDAP with nearly 40 users. Two of those are unable to upload anything through the web interface 'cos an error claims “insufficient storage”. On the admin logs I have the same messages but I’m using nextcloud 11.0.2 (now 11.0.3). The default quota is 2GB but that error appeared to a user without any file on his personal area (1MB space occuped on the directory).
I had to set his quota to “unlimited” to permit him to use nextcloud.
The other users are working ok with no problems.