We are talking about existing /boot/efi which is already flagged. So Calamares should have boot flagged pre-selected. On this one computer it does not. And selecting boot flag manually does not change anything. I was already aware of all of this about the boot flag and so forth.
However, I have changed my mind about this being something that "Must be fixed ASAP". This issue is most likely only going to happen to very few users that are multi-booting on old EFI computers. The "Error dialog" is in fact correct as far as I can tell. If you continue past that point with install the install will most likely be successful but users /boot/efi will have been deleted and created again by Calamares. That is what the logs show. This is the installer log: Calamares-installation.log - Pastebin.com
Little more info if needed. I have 3 computers for testing at the moment. This issue does not happen on 2 of them.
The one it does happen on is by far the oldest one and goes back to when EFI boot was somewhat new. It is an ASUS K53E.
Hardware info for the computer with this issue:
$ inxi -F
System: Host: ben79-2485-K53E Kernel: 5.1.9-desktop-1omv4000 x86_64 bits: 64 Desktop: KDE Plasma 5.15.5
Distro: OpenMandrivaLinux 4.0 Nitrogen
Machine: Type: Laptop System: ASUSTeK product: K53E v: 1.0 serial: <root required>
Mobo: ASUSTeK model: K53E v: 1.0 serial: <root required> UEFI: American Megatrends v: K53E.221 date: 08/10/2012
Battery: ID-1: BAT0 charge: 31.6 Wh condition: 32.7/37.8 Wh (87%)
CPU: Topology: Dual Core model: Intel Core i5-2450M bits: 64 type: MT MCP L2 cache: 3072 KiB
Speed: 798 MHz min/max: 800/3100 MHz Core speeds (MHz): 1: 798 2: 798 3: 798 4: 798
Graphics: Device-1: Intel 2nd Generation Core Processor Family Integrated Graphics driver: i915 v: kernel
Display: x11 server: OpenMandriva X.org 1.20.5 driver: modesetting unloaded: fbdev,vesa resolution: 1366x768~60Hz
OpenGL: renderer: Mesa DRI Intel Sandybridge Mobile v: 3.3 Mesa 19.1.0
Audio: Device-1: Intel 6 Series/C200 Series Family High Definition Audio driver: snd_hda_intel
Sound Server: ALSA v: k5.1.9-desktop-1omv4000
Network: Device-1: Qualcomm Atheros AR9485 Wireless Network Adapter driver: ath9k
IF: wlp2s0 state: up mac: 00:08:ca:71:72:19
Device-2: Qualcomm Atheros AR8151 v2.0 Gigabit Ethernet driver: atl1c
IF: enp4s0 state: down mac: c8:60:00:33:81:56
Drives: Local Storage: total: 465.76 GiB used: 9.43 GiB (2.0%)
ID-1: /dev/sda vendor: Samsung model: SSD 860 EVO 500GB size: 465.76 GiB
Partition: ID-1: / size: 19.56 GiB used: 6.54 GiB (33.4%) fs: ext4 dev: /dev/sda3
ID-2: /home size: 9.78 GiB used: 563.0 MiB (5.6%) fs: ext4 dev: /dev/sda4
ID-3: swap-1 size: 4.00 GiB used: 0 KiB (0.0%) fs: swap dev: /dev/sda2
Sensors: System Temperatures: cpu: 45.0 C mobo: N/A
Fan Speeds (RPM): cpu: 2200
Info: Processes: 163 Uptime: 22h 53m Memory: 3.63 GiB used: 831.0 MiB (22.3%) Shell: bash inxi: 3.0.32f
Again I will get a proper bug report done on this but I am slammed with other things outside of OMA right now.