Did you ever find a solution? Because the same is happening here with 29.0.6
The only thing I can think of is one of those dirs mentioned in the code resolves not to the ncdata but to the www/nextcloud source files, which are not owned by www-data. This is only a guess, and I can’t find it. Would be nice if the error message would say the location of the problem, rather than the location of the error message.