Here's what I did, @scachemaille: I replaced
fontconfig respectively. I also replaced
cairo. I then upgraded
harfbuzz-icu to the latest versions (remember, I had previously downgraded them). Doing all of this solved the problem with
oblogout-manjaro, as you suspected it would. However, the system was ridiculously slow!
Following that, I rebuilt the font cache from scratch (made a backup of the original) using the
fc-cache -vf command in the terminal. I did not witness any observable speedup to the system afterwards, though.
Almost two minutes from logging in to see the desktop. Another 35 seconds for the Tint2 panel to appear, 48 seconds for Conky to appear. It took 11 seconds for terminal to launch, 12 seconds for Thunar file manager to launch, etc. Every application took on average 11 seconds to launch, even
oblogout-manjaro took 20 seconds! I timed these apps at least two times each just to verify.
After that, I reinstalled
freetype2-ubuntu (no longer available in the AUR repo),
cairo-ubuntu. I also downgraded the two
harfbuzz packages again and rebooted. Everything logged in and launched lickety-split just as they should. And the surprise bonus was that
oblogout-manjaro continues to work!
Still, after all of this, icons are still broken. Don't know what to do about that. Reinstalling them doesn't work.