Custom Permlinks work, regular page permalinks don’t

Most likely, you have created custom permalinks that are in conflict with default WP permalinks. Unfortunately, WP doesn’t validate permalinks rules, and it is very easy to create conflicted permalinks rules.

Rules have priority hierarchy, and when resolving any request, WordPress goes through the list of rules, until it matches first one. It is possible that you have created rule that has overridden the default rule so that normal pages depending on that rule can’t be resolved anymore.

You need to be very careful with permalinks rule, especially not to replace default rules without suitable replacement.