Security level settings can be a problem, but the "message" (403) is supposed to mean that you're not authorized to go where you are trying to get to. (or is that "to where you're trying to get"?) Since the site you're connecting to has some control over what messages get displayed, you sometimes do have to "interpret" the messages, and "errors" are common. The 403 error is reported as being used often where the 404 error shoud be used.
Most sites where you need to log in will display a "login" or "logout" button in the header. (Mudcat is sort of an exception, since most of us stay logged in perpetually.) You might check and see if there's a "login" button displayed on the site you're trying to get from/into, since usually it disappears if you are actually logged in and gets replaced by a logout button. If you see a login button, you probably need some additional qualifications. If security settings apply, they may have offered a cookie that your security settings didn't accept. Some sites allow a "guest login" that doesn't grant full access. Look for a FAQ, or info on "membership."
Because of the error that results from an extraneous "/" or other "end characters," if you're using a posted link sometimes right clicking the link, copy shortcut, paste the shortcut into your address bar, and deleting the last character in the address before you hit Enter may get you "somewhere." Sometimes where you get won't be where you want to be, but applying a little "inventive thinking" may lead you on to better things from there.
Sometimes, if you only want a posted "image," right clicking a link to the page and selecting "save target as" will get you stuff that you're not actually "authorized" to see. If you use the link and actually go to the page, they won't let you save it, or sometimes even to see it; but the "save target" sometimes works from outside the actual page where the stuff is displayed. (Only sometimes, but often enough to be worth a try.) Note that even if it works, you still have to figure out where it got saved and find a way to make it useful for your own needs.
The most likely is that the "403 error - access denied" that's been reported to you really should have been a "404 error - page not found," but playing around with it can sometimes get what you're looking for. Be creative.