Ozone programming STM32G4 board, error = 4

Quote from JayTT

When I try and debug using Ozone on a custom STM32G4 board with my JLink Plus Compact, I get:

J-Link: Flash download: Total: 2.351s (Prepare: 0.081s, Compare: 0.267s, Erase: 0.993s, Program: 1.008s, Verify: 0.000s, Restore: 0.000s)
J-Link: Flash download: Program speed: 95 KB/s
J-Link: Flash download: Bank 0 @ 0x08000000: 1 range affected (98304 bytes)
J-Link: Flash download: Total: 2.032s (Prepare: 0.002s, Erase: 1.022s, Program: 1.007s, Verify: 0.000s, Restore: 0.000s)
J-Link: Flash download: Program speed: 95 KB/s
Programming failed (error code = 4) for block @ address 0x08016000 (61)
Download failed: error during program/erase phase
Elf.GetBaseAddr(); // returns 0x8000000
Target.ReadU32 (0x08000000); // returns 0x4, data is 0x20007140
Target.SetReg ("SP", 0x20007140, FALSE);
Target.SetReg ("SP", 0x20007140, FALSE): Program is running.
Elf.GetEntryPointPC(); // returns 0x8002DA4
Target.SetReg ("PC", 0x8002DA4, FALSE);
Target.SetReg ("PC", 0x8002DA4, FALSE): Program is running.

What does this error mean?

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I see that JLINKARM_EndDownload function from Jlink dll have a return value of -4 which says: "Error during verification phase."
From the examples I see that it's used to actually download the data into the flash memory.
Do you have a verify function implemented on your flash loader?