This started me happening out of the blue though it was not the first time...only the first time with this friend.
I start an IM converstion and never get a reply, only to realize on the next day (when the IM works, but using Lumiya this time) through copy and paste of my friend's log...she was answering me all along but the IMs never reached me.
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At first i though it was Radegast, i am using the 2.25 version...However now it also happened while using Firestorm 6.0.2 (56680) Feb 9 2019 18:55:39 (64bit), after hours before the IM working alright through Lumiya.
This has happened to me before long time ago with a different...but only twice or so.
Here's my spec out of the About Firestorm