Hi everyone,
If anyone else has noticed the same behavior…
Kontact can crash when it’s closed (this always happens here), when you change calendar colors, or even when you select or deselect calendars. This is a freshly set up Kontact/Akonadi profile (KDEPIM wasn’t installed beforehand). The system here was updated from Kubuntu 24.04 to 26.04.
Since 26.04 uses SQLite for Akonadi instead of MySQL, I switched to the MySQL/MariaDB backend for testing. But that didn’t make a difference.
I then tested the whole thing on a fresh Kubuntu 26.04 installation in a VM, and it didn’t crash there. It could just depend on quite a few factors.
But at least I’m not the only one; there’s also a bug report:
Many greetings 
Operating System: Kubuntu 26.04 LTS
KDE Plasma Version: 6.6.4
KDE Frameworks Version: 6.24.0
Qt Version: 6.10.2
Kernel Version: 7.0.0-14-generic (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 7800X3D 8-Core Processor
Memory: 64 GiB of RAM (62.5 GiB usable)
Graphics Processor: AMD Radeon RX 7900 XTX
Manufacturer: ASUS