Installation on QNAP - Request Forbidden

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I have an old QNAP TS-439 Pro II, running QTS v.4.2.6 (2020/01/09).
I just cleared everything off of it, and wanted to run Concrete5 as a dev server.
Using the QTS App Center, I installed the .qpkg for C5 (turns out to be v.5.6.1.2.0). Installation says it was successful, and I see the /concrete directory in the Web share.

But when I click on Open, or try to visit the IP of the NAS, I get the error
Error response
Error code 403.
Message: Not Serving Client xxx.xxx.xxx.xxx.
Error code explanation: 403 = Request forbidden -- authorization will not help.


where the IP address is my desktop I'm trying to use to access the site.

I found a few mildly similar posts, and have already tried chmod -R 755 for the web directory. Also looked for an errant .htaccess file... nothing.

Why can't I get to my install??

Thanks!