Win7x64 Ultimate, LibreOffice Version 184.108.40.206.beta2 (Build ID: f010139)
Open a new spreadsheet.
In any cell, insert a condition in Conditional Formatting, [Ok].
Edit the conditional formatting for this cell, nothing there.
No problem with LibreO 220.127.116.11
Sounds like a DUP of "Bug 51555 - FORMATTING Not possible to add Conditional formatting"
Please feel free to reopen this Bug if you find evidence that we have an independent issue here that still exists in 18.104.22.168.beta3.
*** This bug has been marked as a duplicate of bug 51555 ***