Add the known problem with the missing packages after download

docs/shared/elevate/README.md

@@ -129,7 +129,7 @@ Please refer to the following documentation links for instructions on how to do
 
 * [How to Backup & Migrate All CloudLinux Settings to Another Server](https://cloudlinux.zendesk.com/hc/en-us/articles/115004277894-How-to-Backup-Migrate-All-CloudLinux-Settings-to-Another-Server)
 * [How to Restore PHP Selector Options](https://cloudlinux.zendesk.com/hc/en-us/articles/115004517465)
-* [How to obtain activation keys, register and migrate servers ](https://cloudlinux.zendesk.com/hc/en-us/articles/115003888573-How-to-obtain-activation-keys-register-and-migrate-servers)
+* [How to obtain activation keys, register and migrate servers](https://cloudlinux.zendesk.com/hc/en-us/articles/115003888573-How-to-obtain-activation-keys-register-and-migrate-servers)
 
 #### I have a CentOS 7 system I want to migrate to CloudLinux 8, can I use your tool to do so?
 
@@ -276,7 +276,7 @@ If not, enable the corresponding package repository (e.g. `centos-extras`) and i
 `dnf install 'dnf-command(config-manager)'`
 
 
-#### DNF transaction failure
+#### DNF transaction failures
 
 The main upgrade transaction is performed while the system is booted into a custom InitRamFS. From there, all the package operations prepared during the previous steps are performed.
 
@@ -288,6 +288,27 @@ For example, if a package encounters a fatal error inside its `%preun` or `%prei
 
 It is recommended to remove such packages prior to the upgrade, or, alternatively, add them to the `to_keep` list (see *Transaction Configuration Files* section) to prevent Leapp from attempting to upgrade them.
 
+#### Missing upgrade package files
+
+In some cases, you may find the upgrade process halting at the main upgrade DNF transaction with a list of error messages similar to the following:
+
+```
+2024-01-01 17:24:55.396 DEBUG    PID: 1053 leapp.workflow.RPMUpgrade.dnf_upgrade_transaction: Error opening /var/cache/dnf/almalinux8-baseos-f11d00b8ca777dac/packages/alt-php82-newrelic-10.16.0.5-1.el8.x86_64.rpm: No such file or directory
+2024-01-01 17:24:55.410 DEBUG    PID: 1053 leapp.workflow.RPMUpgrade.dnf_upgrade_transaction: Package "alt-php82-newrelic-10.16.0.5-1.el8.x86_64" from repository "almalinux8-baseos" has incorrect checksum
+```
+
+In the known possible error scenario, this problem occurs when the upgrade packages were downloaded from CLN-provided package repositories, but CLN was then disabled due to a registration/networking error encountered.
+
+Check the version of the dnf-plugin-spacewalk being installed into the isolated upgrade environment in the logs. Its release must be either `cloudlinux.2.leapp.2`, or `cloudlinux.5` and above for the CLN/upgrade integration functionality.
+
+Also, make sure that the following line is present:
+
+```
+leapp.workflow.RPMUpgrade.dnf_upgrade_transaction: Leapp upgrade is running - using cache.
+```
+
+If neither of those appear to be the problem, its source may be different from the known one. Please contact support for further assistance.
+
 
 ### ELevate cPanel Scenario issues