JabRef version 3.8.1 portable (.jar file) on Windows 7 Pro and Ubuntu 16.04

To summarise, I can view large database (>1000 references, bib file > 2.5 MB) but cannot edit it with version 3.8.1. The same database can be edited with version 3.8.

Version 3.8.1 can only edit small database.

Steps to reproduce:

1. create large database (mine is 2.52 MB bib file containing 1080 references of articles)
2. open the file with JabRef 3.8.1 portable (.jar executable)
3. I can view the references but cannot enter edit mode (hit enter key, click Edit Entry button in toolbar, double click any entry, ... Nothing works). But the same database can be edited with version 3.8 portable.
4. create small database (mine is bib file with 14 KB containing 32 articles.)
5. version 3.8.1 portable works perfectly.

If it helps, I also tried version 3.8.1 (.exe file) installed in Windows 7 Pro. Same problem for large database.