Sometimes multi-layer ceramic capacitors can go resistive if they've been subject to physical stress (cracking) or contamination such as corrosion damage from capacitor electrolyte. It's not uncommon for this kind of fault. If the capacitor appears damaged/corroded or it there's any sign of heating, it should be replaced.
The valid clue here is that the 'insert disk' animation is getting screwed up. The fact that the rest of the graphics mainly appear to be drawn OK suggest that the graphics output hardware (U4, U12, etc) is likely OK.
I'm not sure exactly how the disk animation works at the software level, but in hardware it relies on the level 3 interrupt (_INT3 on the schematic). This is generated by U2/Agnus, which gets passed to the interrupt handler U3/Paula; presumably the software is relying on the interrupt to move the disk image data between different areas of memory to create the animation, not sure. Based on the clues in the screen photo, something about this interrupt is broken.
While viewing the 'insert disk' screen, monitor what's happening with _INT3. The line should be inactive all the time (high), and only strobes active (low) while the disk is 'moving', with an active duration of around 200nSec. Make sure this signal is arriving at U3/Paula pin 20. If that appears to be OK, there's possibly something broken with the interrupt priority level lines from U3/Paula and the 68000, so check continuity of the three IPL lines between U1 and U3.
