I'm running 23.0.91 MediaCenter. I can remotely connect to it just fine with a v23 client. However, I am unable to connect to it with my father-in-law's v21 client with either my access key or URL. Entering my external IP:52199 in a browser from his laptop pulls up my library just fine so it isn't a networking issue. Also I'm connecting with my v23 client from the same network further ruling out a networking issue. I've tried generating a new access key, and reinstalling his v21 client. What appears to be the relevant portion of the log is below.
0039828: 576: Reader: CWinINetReader::Open: Opening http://pool-<IP HERE>.bstnma.fios.verizon.net:52199/MCWS/v1/Library/Get
0039828: 1076: Reader: CWinINetReader::Thread: Start
0039828: 1076: Reader: CWinINetReader::Connect: Start
0039828: 1076: Reader: CWinINetReader::Connect: Finish (0 ms)
0039828: 1076: Reader: CWinINetReader::DownloadFromHTTPURL: Start
0040359: 1076: Reader: CWinINetReader::DownloadFromHTTPURL: Success
0040359: 1076: Reader: CWinINetReader::DownloadFromHTTPURL: Finish (531 ms)
0040359: 1076: Reader: CWinINetReader::Thread: Finish (531 ms)
0040359: 576: Reader: CWinINetReader::Open: Finish (531 ms)
0040469: 576: Reader: CWinINetReader::Close: Start
0040469: 576: Reader: CWinINetReader::Close: Finish (0 ms)
0040500: 576: Database: MCDB::Load: Loading from path:
0040500: 576: Database: MCDB::Load: Failed
0040500: 576: Database: MCDB::Unload: Start
0040500: 576: Database: MCDB::Unload: Stopping worker threads
0040500: 576: Database: MCDB::Unload: Getting database lock
0040500: 576: Database: MCDB::Unload: Starting unload
0040500: 576: Database: MCDB::Unload: Updating child objects after unload
0040500: 576: Database: MCDB::Unload: Resetting info
0040500: 576: Database: MCDB::Unload: Finish (0 ms)
0040500: 576: Database: MCDB::Load: Finish (5500 ms)