Hi, just checked and found it's working as expected. Also I'm using:
ReminderFox [2.1.6.3] reminderfox_2.1.6.3b_debug_20180731_1418.zip
Profile directory: /media/guenter/DATA/_Mozilla/TB_gW/Profiles/201603.GW.56
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 (en-US)
Maybe first you can upgrade to TB 52.9.1
If the problem is there also, please manually update RmFX (see below)
That's a debug version I'm trying to fix a different problem. To install please go for the following steps:
1. Open the Thunderbird Add-ons Manager page:
If the 'Menubar' with "File ... Tools Help" isn't shown, press F10
Use [Tools] --> Add-ons
2. Using Firefox open the page
but make sure you see the TB Add-ons tab
4. Finally you need to restart TB.
Hope you have fixed your problem, if not please call back.
Günter