We experienced the same issue with one of our applications, the following article guided us: CreateSearchIndexOnReload;
You will find this parameter initialized to 1 (yes) in the Data Load Script of the offending application(s), just turn it off, and it will reload, even faster, if you read the article above, that index is required when you perform searches on the User Interface, we haven't experience any issue since turning it off on the applications returning the error when reloading.
Hope this helps,
As stated above, you can turn off this feature and give a try on your reload.
While the technical reason for such failure is not known, I can explain the nuances behind CreateSearchIndexOnReload.
This parameter when enabled instructs Qlik to build indexes for the various fields once reload is complete.
Disabling this would not cause any impact to reload process. It is just that the first search you do using the search option on the UI would take an additional overhead of building indexes at the runtime, which might be even negligible depending on your application.
Hope this helps!