I set my progress bars to a certain texture. When I quit the game and restart, the default progress bar texture is being used, not the one I chose. I can then set the texture again (it's still available) but I have to redo that each session. It used to work just fine but since a couple days this bug occurs.

Edit: Might have first appeared in the 22nd April update that I downloaded from Curse/Twitch.

After downloading/deleting/re-installing WeakAuras it seems to be working again now. Coincidentally I removed GSE (Gnomish Sequencer Addon) at the same time. So maybe that was the actual culprit. Will try to reproduce.

The most likely reason for that is that you are using a texture that is registerd by a addon, but that addon registeres the texture not immediately on loading.