this is a newer error customer reports, over last 2 months

While I did recently update this client's cmsb version (to 3.84), he says he's had this challenge for about 2 months (version 2.77).  Sometimes, not every time, when he's uploading a lot of images at one time the browser/system hangs and locks up.  Initially I put it off as a glitch, or the hosting company at those times was running tight on resources or had some temp issue.  It just happened again, about an hour ago, but the error on the backend seems a little different than past ones so I'm writing in to see if there's something that can be done.

On the error list page (modified for privacy/security):

#135579 - Unable to remove file '/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp' Please ask your server administrator to check permissions on that file and directory. The PHP error message was: unlink(/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp): No such file or directory
/home/xxxxxx/public_html/cmsuAMGP383/lib/upload_functions.php:679
POST https://xxxxxxxxxx.com/cmsuAMGP383/admin.php

and inside the details of the error, the mention of the "upload strategy" file caught my attention:

Backtrace

#0 Script started                                         at /home/xxxxxx/public_html/cmsuAMGP383/admin.php:0
#1 require()                                              at /home/xxxxxx/public_html/cmsuAMGP383/admin.php:82
#2 require()                                              at /home/xxxxxx/public_html/cmsuAMGP383/lib/menus/default/actionHandler.php:106
#3 require()                                              at /home/xxxxxx/public_html/cmsuAMGP383/lib/menus/default/uploadForm.php:4
#4 submitUploadForm()                                     at /home/xxxxxx/public_html/cmsuAMGP383/lib/menus/default/uploadForm_functions.php:24
#5 removeExpiredUploads()                                 at /home/xxxxxx/public_html/cmsuAMGP383/lib/menus/default/uploadForm_functions.php:38
#6 DefaultUploadStorageStrategy->removeUploadsForRecord() at /home/xxxxxx/public_html/cmsuAMGP383/lib/upload_functions.php:679
#7 RuntimeException thrown                                at /home/xxxxxx/public_html/cmsuAMGP383/lib/upload_storage_strategy.php:130
   -> Unable to remove file '/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp'

Please ask your server administrator to check permissions on that file and directory.

The PHP error message was: unlink(/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp): No such file or directory

and 
RAW LOG DATA: 
errfile          => "/home/xxxxxx/public_html/cmsuAMGP383/lib/upload_storage_strategy.php"
errline          => 130
errno            => "RuntimeException"
errstr           => "Unable to remove file '/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp'\n\nPlease ask your server administrator to check permissions on that file and directory.\n\nThe PHP error message was: unlink(/home/xxxxxx/public_html/images/uploads/thumb2/35-img-6665-jpg.webp): No such file or directory\n"
exceptionClass   => "RuntimeException"
exceptionCode    => 0
externalFilePath => "/home/xxxxxx/public_html/cmsuAMGP383/lib/upload_functions.php"
externalLineNum  => 679
logType          => "exception"

 So I am just wondering (because I'm still a programming neophyte) if the "upload strategy" file has some limiter in this case, or something?  Or is it just a challenge with the hosts' resources?

It seems when he's uploading around 100 images at one time it occurs, and if he has a problem, he gets around it uploading 20 at a time (but it drives him crazy).  I will mention here that there was a similar challenge that was worked out several years ago and he does have a plugin (built by IT) for resorting the images after saving. 

Thank you for your attention to this.