ok, couple more sanity checks - pointers - so to speak.
1. make sure that Alt-F Samba config has SMB1, SMB2, or both (better both as Windows 10 doesn't have SMB1 enabled by default, if I recall correclty) checked and click submit, reboot DNS, try to access from Windows box (reboot it if you haven't already done that since the problem occurred).
This is to make sure that Alt-F Samba service runs and nothing stalled.
2. from what I've googled "121" and "semaphore timeout" are indication of Windows box not talking to DNS, most likely on DNS side as DiskStation talks. Could be hardware issue - pull and switch cable leading to DNS, use different port on Router (from DiskStation may be). See if this helps.
3. Alt-F 0.1RC4 is an outdated release, current - Alt-F 1.0. Just for your information. Not sure if this would be a good moment to consider an upgrade... as the problem hasn't been localized yet, I have DNS-323-B1 with ALT-F 1.0 running on it, everything works flawlessly for now.