VB 6.0.0: guest WinXP crash when using serial port (W10 host - Wxp guest)

[dx7]

Hello.
I use WinXP as a guest OS. When I launch an application that uses serial port communication, usually after 30-60 minutes in average, the error dialog shows up and then the VM crashes.
The serial port is an on-board RS-232 port (not USB-RS232 adapter). The port works fine in the host OS.
When I use the USB-RS232 adapter, VM doesn't crash and everything seems to work fine.

I attached a whole report made with VBoxBugReport and the log itself so I hope that helps.

And I also noticed other thing. In terms of the serial communication, there are ping request and response messages defined in the application I'm running on VM.
When ping request msg is sent, the response msg is supposed to arrive almost immediately (200 ms max). But it takes longer.
Exact behaviour: I send a ping request every 500 ms, so in 1500 ms I send 3 pings. After 1800 ms roughly, all 3 ping responses arrive all at once.
When I use USB-RS232 adapter, the ping response time is normal. I just thought it might've helped you.

VirtualBox v5.2.18 didn't have these issues.

PC info:
- Intel Xeon CPU E3-1240 v5 3.5GHz
- 16 GB RAM
System info:
- VirtualBox 6.0.0 r127566 (Qt5.6.2), Extension Pack 6.0.0.r127566
- Host OS: Windows 10 Pro x64 v1803 (OS assembly 17134.472, last update Jan 09 2019)
- Guest OS: Windows XP Proffesional x32 (Version 5.1 build 2600.xpsp_sp3_gdr.130307-0422: Service Pack 3)

Thanks