Hello,
a little bit late, but maybe not to late.
I had the same issue. The first sync was fine, but after that, I couldn’t sync anymore. After month of searching I find the problem.
I use nginx. In the documentation of nextcloud 10 the following lines are written:
error_page 403 /core/templates/403.php;
error_page 404 /core/templates/404.php;
after commenting them out, syncing works fine.
If the problem still accords, change
fastcgi_intercept_errors on;
to
fastcgi_intercept_errors off;
and test syncing again. Maybe that a hint for the real problem.
Greetings,
Daniel