Not sure if this is entirely an issue with avastUI.exe as the faulting module is ntdll.dll and it may gust have been that the avastUI.exe was using it at the time of the crash, or do you mean BSOD ?
If it is a bsod - Upload any minidump or memory.dmp files, zipped to reduce size. Give the zip file you are uploading a unique name (e.g. forumusername-mem-dump.zip, etc), so they can identify it. It might not be a bad idea to create a text file (readme.txt) with any relevant information, avast topic URL, user name, etc. etc. in the zip file.
Memory dump locations, Mini Dump files in, C:\Windows\Minidump\ - Full Kernel dump file, C:\windows\memory.dmp
Using Internet Explorer, Connect to the link and drag the file into the Right pane and drop it, that starts the upload, you don’t have read access to this folder.
Or
Upload it using the Run command-line in Windows: Windows Key + R (to get the run box), copy and paste this [nobbc]explorer ftp://ftp.avast.com/incoming[/nobbc] and drag the file into the window, from another explorer window.
I do not get a BSOD. Avast crashes and I get the message that this has happened. If I restart Avast, same thing happens within a few minutes. I have been running Avast for quite a while without problems. I have it running on a laptop that I have not upgraded to Ver 6.x and it runs fine. This only happened when I upgraded Avast last weke.
If avast crashes, I don’t know if you can do a crash dump from the task manager if the AvastUI.exe is still in the task manager, Processes list you are meant to be able to right click on it and select Create dump file.
If that is possible then perhaps you can upload that for analysis.