I print ABS with 100 deg bed and 240 deg hot end, usually at 40mm/s. At 55mm/s I might up the hot end temp a little more, but 260 sounds too hot. I use mostly ABS from rigid.ink; i think it prints a few degrees cooler than some others, but still 260 sounds high. try lowering it to 250 as a start, and maybe more than that.
Also maybe start slower and work the speed up. Is there any grinding in the extruder? Also what retraction settings do you have?
Slow speed down to 40-45 play with temp each color and is a little different I put a acrylic door on ultimaker with pet over glass bed I get my filament from matterhackers I only print abs and carbon fiber I have the iroberts feeder
As far as raft is concerned - I’d stay away from that. That’s an old technology from before Cura was created and cura doesn’t do it well and we have newer, better technologies. Raft was helpful back in the days before we had heated beds. To get your part to stick to the bed here is a thorough summary: How to get your 3d printed parts to stick to the bed and avoid curling/warping - YouTube 3
I think that could be the case because you can see the underextrusion appears not only at the bottom. In general if you just cleaned your nozzle it will start without a problem, but if you not cleaned the nozzle problems from the start. Than you know you need a replacement.
Another possibility is that the spring of the extruder is too tight and your filament becomes oval instead of round. The oval dont fit in the bowdentube, you can feel the friction by hand or you can measure it.
Especially the first layer needs a very slow printing speed.for good adhesion. I use PAI now for good bed adhesion works better than buildtak. (to bay as a spare part for reprap printers)
The fans on the UM3 have rpm control or something weird. 30% and even 10% on the UM3 is the exact same speed as 100%. Only when you get below 10% does it start to slow down. I would do 3% fan. Or even lower. Start with 3%. It broke half way most likely due to too much fan.
So you were getting underextrusion near the end. Not sure why. Were you using cura recommended ABS print temperature? I recommend sticking to cura recommendations for UM3 as they tuned the ABS reasonably well.
Did you cover the front and/or the top of the printer? It really helps a lot.
The fans alone might explain both issues - not sure - underextrusion towards top as it is farther from the print bed and bad bonding.
I think I already asked this but can you get by with PLA? PLA is soooo much easier to deal with. PLA will be stronger, easier to print - the only down side of PLA versus ABS is that PLA can’t handle temperatures above 50C. That’s hot enough to kill a human but is still a common temperature for a car sitting in the hot sun in the summer. So if this is something that needs to be in boiling water or is used in an automobile then definitely don’t use PLA.
Unfortunately I’m really not sure what is causing your underextrusion at the top. Maybe a filament tangle? Check to see if the filament on the rear spool is all tangled or caught on something maybe?
Another thought - 50mm/sec is a bit fast if you are doing .2mm layer height. 0.1mm layer height at 50 is fine but if you are doing .2mm and 50mm/sec I suspect you should try lowering that to maybe 35mm/sec. If you are watching it print and see that characteristic holes-in-the-side of the print as you see in the upper area of the print then just go to TUNE and lower the speed to say 50%.