Dell monitor not recognised

I just tried to install the beta on my rasp pi 3 b+.
On my dell monitor, it said "cannot display this video mode, optimum resolution 1280*1024 at 60hz".
i don't have the rainbow screen.
If i do it on my video projector, it's working fine. But it's not where i want to use it.
With raspbian, no problem on this monitor....
With xubuntu, no problem ( (Unofficial) Xubuntu-core armhf - https://1drv.ms/u/s!AvHY_kl4hMB4gQIR4CLRKQzh-GP0

Can you help, do you know this problem?
maybe something to improve on the next beta....
Thanks anyway, the little i tried, looks good...

You might get more help if you said which Dell monitor,and which beta version.
As I sit here, I can see my installation on a Pi3B+ with a Dell monitor and the tvservice command shows 1280x1024 at 60Hz.
I had no instal problems, but did have to fix an SSH problem (known).
Derek

It's a dell 1708fp.
i tried to change the cmdline but nothing more....

i tried with the 2 beta 18.04.2 for armhf and armv8.
absolutely the same problem

Looking at the specs for that, it shows 1280x1024 @75Hz,
and I see 2 types of connector: DVI and VGA
I use VGA with a suitable HDMI converter on my older Dell monitors
HTH
Derek