Hi Shivam,
I checked, and run the indexing for 2 hours. The server gets exhausted from indexing a few longer documents consecutively (15 000+ words per document). Looks like it is able to finish them, but it takes a lot of time, and sometimes the server kills the process or restarts it.
The plugin tries to slow down the indexing accordingly, but after a few failures it does not continue, to not trigger more errors.
I was able to get to around 83% indexed documents. All you need to do if the error appears, is to simply refresh the page, wait a bit, and then hit the “Continue existing index” button. It should not happen often though. Since there are a lot of documents, this will take some time, but should finish eventually without any issues.