Oh YAY, i've solved the blank black thumbnails problem for 5.8.0 in Activities Manager.
Credit goes to Cobber in https://forum.kde.org/viewtopic.php?f=30...73#p365647, from whose good detective work i had my idea. S/he discovered that the Activity Pager widget is the trigger for the problem, & removing it leads to the problem going away [after logout/in]. Even though s/he was troubleshooting 5.8.1 in KDE Neon, & i still "only" have 5.8.0 in Maui, i thought it might be worth testing. I deleted this widget, logged out/in, & TA-DA, the A/M now shows the correct thumbnails for my three Activities again, just like it used to in 5.7.5. Cool.
PS -- I'm terribly sorry for not originally mentioning that i had put that widget into my Task Bar; never in a million years would i have guessed that it could be relevant to the problem root cause. Sigh.
Credit goes to Cobber in https://forum.kde.org/viewtopic.php?f=30...73#p365647, from whose good detective work i had my idea. S/he discovered that the Activity Pager widget is the trigger for the problem, & removing it leads to the problem going away [after logout/in]. Even though s/he was troubleshooting 5.8.1 in KDE Neon, & i still "only" have 5.8.0 in Maui, i thought it might be worth testing. I deleted this widget, logged out/in, & TA-DA, the A/M now shows the correct thumbnails for my three Activities again, just like it used to in 5.7.5. Cool.
PS -- I'm terribly sorry for not originally mentioning that i had put that widget into my Task Bar; never in a million years would i have guessed that it could be relevant to the problem root cause. Sigh.