If you have troubles since the update to WordPress 4.5 please read this post before opening a new thread.
Once you adjust your permalink structure the rewrite rules should be flushed and stop this problem.
If, however you have another page (apart from the Custom Post Type which displays your Portfolio entries) you have another page / category with the name “portfolio”, this will cause 404 errors.
Check there are no other pages / posts / categories with the same name, if there is delete it (and empty trash) then re-save your permalink structure (this will re-flush the rewrite rules), hopefully fixing the problem.
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