Hey lafleurengine, could you please record a net-log dump of this problem? Instructions here:
https://dev.chromium.org/for-testers/providing-network-details

Is this a recent regression with the upgrade to Chrome 66?

https://files.fm/u/4jhc99n5  (wasn't able to upload here, over 10 MB)

"Is this a recent regression with the upgrade to Chrome 66?"
I'm not sure.

Hmm, link broken, sorry for the slowness getting there..

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Re-uploaded: https://files.fm/u/fxzc8daz

Thanks. All mentions of this error code seem to look roughly like this (seem to be on Facebook CDN):

3914: URL_REQUEST
https://static.xx.fbcdn.net/rsrc.php/v3iYKF4/ya/l/en_GB/zx1IF08OhSt.js
Start Time: 2018-06-05 18:26:44.587

t=67056 [st=  0] +REQUEST_ALIVE  [dt=194]
                  --> priority = "LOWEST"
                  --> url = "https://static.xx.fbcdn.net/rsrc.php/v3iYKF4/ya/l/en_GB/zx1IF08OhSt.js"
t=67057 [st=  1]    URL_REQUEST_DELEGATE  [dt=0]
t=67057 [st=  1]   +URL_REQUEST_START_JOB  [dt=159]
                    --> load_flags = 17216 (DO_NOT_SAVE_COOKIES | DO_NOT_SEND_AUTH_DATA | DO_NOT_SEND_COOKIES | MAYBE_USER_GESTURE)
                    --> method = "GET"
                    --> url = "https://static.xx.fbcdn.net/rsrc.php/v3iYKF4/ya/l/en_GB/zx1IF08OhSt.js"
t=67057 [st=  1]      URL_REQUEST_DELEGATE  [dt=0]
t=67057 [st=  1]      HTTP_CACHE_GET_BACKEND  [dt=0]
t=67057 [st=  1]      HTTP_CACHE_OPEN_ENTRY  [dt=2]
t=67059 [st=  3]      HTTP_CACHE_ADD_TO_ENTRY  [dt=11]
t=67070 [st= 14]      HTTP_CACHE_READ_INFO  [dt=30]
t=67101 [st= 45]      HTTP_CACHE_READ_INFO  [dt=60]
t=67216 [st=160]      URL_REQUEST_DELEGATE  [dt=0]
t=67216 [st=160]      URL_REQUEST_FILTERS_SET
                      --> filters = "BROTLI"
t=67216 [st=160]   -URL_REQUEST_START_JOB
t=67216 [st=160]    URL_REQUEST_DELEGATE  [dt=0]
t=67216 [st=160]    HTTP_CACHE_READ_DATA  [dt=23]
                    --> net_error = -6 (ERR_FILE_NOT_FOUND)
t=67239 [st=183]    FAILED
                    --> net_error = -401 (ERR_CACHE_READ_FAILURE)
t=67250 [st=194] -REQUEST_ALIVE
                  --> net_error = -401 (ERR_CACHE_READ_FAILURE)

Corresponding cache log:
3915: DISK_CACHE_ENTRY
https://static.xx.fbcdn.net/rsrc.php/v3iYKF4/ya/l/en_GB/zx1IF08OhSt.js
Start Time: 2018-06-05 18:26:44.588

t=67057 [st=  0] +DISK_CACHE_ENTRY_IMPL  [dt=230]
                  --> created = false
                  --> key = "https://static.xx.fbcdn.net/rsrc.php/v3iYKF4/ya/l/en_GB/zx1IF08OhSt.js"
t=67073 [st= 16]   +ENTRY_READ_DATA  [dt=0]
                    --> buf_len = 5132
                    --> index = 0
                    --> offset = 0
t=67073 [st= 16]   -ENTRY_READ_DATA
                    --> bytes_copied = 5132
t=67101 [st= 44]   +ENTRY_READ_DATA  [dt=0]
                    --> buf_len = 16
                    --> index = 2
                    --> offset = 0
t=67101 [st= 44]   -ENTRY_READ_DATA
                    --> bytes_copied = 16
t=67216 [st=159]   +ENTRY_READ_DATA  [dt=1]
                    --> buf_len = 32768
                    --> index = 1
                    --> offset = 0
t=67217 [st=160]      ENTRY_DOOM
t=67217 [st=160]   -ENTRY_READ_DATA
                    --> net_error = -6 (ERR_FILE_NOT_FOUND)
t=67287 [st=230] -DISK_CACHE_ENTRY_IMPL

There are some other fetches that just write to cache, and they seem successful.
Need to look at code closer to see what issues those errors.

So this looks like an awful like the call for EntryImpl::GetBackingFile on the read fails (for stream 1 but not stream 0), but not on write. 

Is there a way of getting to LOG(ERROR) on windows? (Maybe launching from cmd.exe or something) --- that may be very slightly enlightening.

@Reporter: The contents of chrome://net-internals/#httpCache might also be somewhat enlightening..

@Reporter: Could you please respond to comment# 9 by providing chrome://net-internals/#httpCache, hence adding Needs-Feedback label.

Thanks!

Sorry for the delay, hadn't received the issue for a while.

For chrome (although not same error - Failed to load resource: net::ERR_SSL_PROTOCOL_ERROR)

And opera (Failed to load resource: net::ERR_CACHE_READ_FAILURE):

Deleted: http cache chrome.txt 1.1 KB

http cache chrome.txt 1.1 KB View Download

Deleted: http cache opera.txt 1.1 KB

http cache opera.txt 1.1 KB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

A Gentle ping...

csharrison@/morlovich@ - As per comment# 11, reporter has provided the chrome://net-internals/#httpCache details as requested in above comments.
Request you to have a look at this issue and provide an update.

Thanks..

That's sadly not very enlightening; the opera log shows zero errors, while the Chrome one does show that some cache entry wasn't written out properly, but that doesn't seem directly related.

Both browsers being affected is very curious, though, since it makes it unlikely that it's just a matter of a corrupt cache directory (though I suppose both getting screwed up in the same way isn't impossible, since they would have many shared bugs!)

About the only thing I can think of that may shed some light on things is to get the error message log (and that only has a somewhat limited chance of helping); there are some instructions on https://www.chromium.org/for-testers/enable-logging
(though the verbose bit isn't needed)

Please note the warnings on privacy there --- the log may have stuff you consider private in it, so you probably don't want to just upload it.
The parts that are of interests are those that mention backend_impl.cc, block_files.cc, entry_impl.cc, storage_block-inl.h

Also note that changing the cache backend to simple via chrome://flags/ may be a workaround (one that will wipe the entire cache, and likely the 
chance of figuring out what's going on, but may be the right thing for you when it comes to getting your things done).