icons in v6.0.3.2 unreadable

I upgraded my Ubuntu to 18.4 and now Libre Calc shows two lines of icons at the top of the page which are undecipherable…

It could be a problem with OpenGL:

https://www.libreoffice.org/download/release-notes/

https://wiki.documentfoundation.org/OpenGL

From the main menu, select Tools:Options:View and select Large on Icon Sizes.