Error converting to utf8mb4

[rllmukforum]

I'm attempting to run the utf8mb4 conversion using the tool baked into the ACP since 4.1.12. It ran for a while with a standard IPS progress bar, then timed out with an nginx error at approximately 40%. If I reload the ACP and try to run the tool again, it resumes earlier in the process, quickly gets to the same point on the progress bar, and then shows the nginx error with a URL ending in:

&app=core&module=support&controller=utf8mb4&mr=81

We have approx. 10m posts, and our test environment has 8 xeon cores with the db cached entirely in the 64gb of memory, and obviously just me logged into it. I was expecting it to take a while, but not error out.

Is it still running, despite the error/inability to resume through the UI? Or must the progress bar be active in the UI? What logs should I look for to determine where it's falling over?

 

edit - the last two attempts to resume through the ACP caused an nginx error with the following URL params:

&app=core&module=support&controller=utf8mb4&mr=0&_mrReset=1

 

[jair101]

I have the same problem, altough with me the error is that mysql has gone away. IPS couldn't help, because it is a test install and I am very reluctant to do this on the live one, because when that happens the search is broken and half your tables are converted and half are not. 

I am also using nginx, so I wonder if this is somehow related.

Let me know if you find a solution, for now I feel I will live without emojis.