Same here, Windows 10.0.18362 Build 18362, tried what @NimbM has mentioned in his post.
Connecting to Freenode via SSL.

Looking at Wireshark, Freenode sends RST packet after receiving the following payload from mIRC:
Code
0000   e4 8d 8c 55 43 cd 74 46 a0 3b 1d 7d 08 00 45 00   ...UC.tF.;.}..E.
0010   00 43 7f b9 40 00 80 06 00 00 c0 a8 02 08 cc e1   .C..@...........
0020   60 fb df 1e 1b 58 03 e7 09 43 4f 36 2a 77 50 18   `....X...CO6*wP.
0030   04 02 f0 c2 00 00 4e 49 43 4b 20 78 78 78 0a 55   ......NICK xxx.U
0040   53 45 52 20 78 78 78 20 30 20 2a 20 3a 2e 2e 2e   SER xxx 0 * :...
0050   0a                                                .


Here is the whole transaction:

[Linked Image from i.imgur.com]

Perhaps it's not exactly in the format Freenode's software is expecting?