I have a CGI tool on Toolforge that provides some reviewing (Flagged Revs) stats for ruwiki, here it is: [1]. Server truncates it response to 64 kb. Looks like here [2] described this or similar problem, so, the cause of the problem is that I don't have a permission to write into temp folder/file. My error.log file contains many of this error messages:

2019-06-08 03:30:48: (chunk.c.553) opening temp-file failed: No such file or directory
2019-06-08 03:30:48: (mod_cgi.c.786) demuxer failed:

[1]: https://tools.wmflabs.org/mbh/patstats.cgi?date1=2019-04-01&date2=2019-06-08 (response generates in ~2 minutes)
[2]: https://stackoverflow.com/questions/10557927/server-response-gets-cut-off-half-way-through