Following latest update, Dolphin refuses to open,
app-org.kde.dolphin@b0176df4ce3e472ab25c3715b363b57e.service: Main process exited, code=dumped, status=11/SEGV
Try logging out of Plasma completely, switching to a tty, and deleting the contents of ~/.cache.
That was quick. Sorry but I’m a bit dumb, how do Iswitch to TTY?
CtrlAltF3. Then log in there as yourself, and at the prompt, issue the following command… 
rm -rf ~/.cache/*
Once you’ve done that, close the tty session with CtrlD, and switch back to the sddm login screen with AltF1 — or possibly AltF2, because sddm doesn’t always run on the same tty.
Same result. Bouncing mini-icon and stops. ksystem log details.
app-org.kde.dolphin@6a4b00901d414b5fbed1a07fc4f66a47.service: Failed with result 'core-dump'.
Also had this in KSystemLog
dolphin[2311]: segfault at 7d022e3e0b90 ip 00007d022e3e0b90 sp 00007ffc13b53a28 error 14 likely on CPU 0 (core 0, socket 0)
and
Code: Unable to access opcode bytes at 0x7d022e3e0b66
That could be a hardware problem, albeit not necessarily a permanent one.
A culprit often overlooked is static electricity or interference from cosmic radiation and/or other electromagnetic influences.
Try powering down the device, unplugging the power cord, and leaving it fully powered down for about 15 minutes. That should clear all electromagnetic contamination.
If however the problem persists, then I would advise you to run memtest86+, and let it run for at least 24 hours. It might not detect anything, but it never throws false positives, so if it does detect something, then you’ve got a failing memory module.
I wonder if this is connected to problems I’ve had with Firefox and Brave over recent weeks? Tabs crashing etc. at random times.
I might pull/replace memory as well. see what happens.
Thanks for your input Aragon. 
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That could indeed very well be the case, yes.