Hello there! I know it's been five years since I last posted about this. I should have posted my solution right away, after I finally had resolved the issue, but since I did not get Last.fm to work with WACUP, I tried, tested and failed countless times, untill I got Last.fm -legacy scrobbler to work with old Winamp. So, I gave up on WACUP (feeling frustrated and slightly put-off for WACUP) and then forgot about it. But I understand that WACUP now fully supports Last.fm scrobbling, or am I mistaken? This means that I can stop using my old Winamp and start finally using WACUP! - I need to test the newest release and see how it works...

However, just in case if anyone is interested... in my case, I got Last.fm legacy scrobbler to work with old Winamp 5 in following way:
1) Install Winamp
v5.666 (This was the latest version I got this thing working with.)
2) Install Last.fm legacy scrobbler
v2.1.37 3) Confirm that Winamp plugins folder has both
in_mp3.dll (should be
v4.103.0.0 - if not, search and download installer for
in_mp3__4_103__winamp5666_3512),
and either
ml_wa2_scrobbler.dll or
gen_wa2_scrobbler.dll in it, either one should work.
4) (Search, locate and download required SSL libraries) and copy DLL -files
libeay32.dll and
ssleay32.dll,
in Windows 10, into
C:\Program Files (x86)\Last.fm\plugins
This has been now working without problems for the past 5 years! Thanks for everyone concerned!