[Plugin: NextGEN Gallery] Bulk actions don't work

Hi everybody!
I use NextGen 1.7.3 on my virtual hosting.
Server settings:

* Operating System : Linux (32 Bit)
    * Server : Apache/2.2.3 (CentOS)
    * Memory usage : 17.95 MByte
    * MYSQL Version : 5.0.77
    * SQL Mode : Not set
    * PHP Version : 5.2.11
    * PHP Safe Mode : Off
    * PHP Allow URL fopen : On
    * PHP Memory Limit : 128M
    * PHP Max Upload Size : 100M
    * PHP Max Post Size : 100M
    * PCRE Backtracking Limit : 500000
    * PHP Max Script Execute Time : 60s
    * PHP Exif support : Yes ( V1.4 )
    * PHP IPTC support : Yes
    * PHP XML support : Yes

Graphic Library

    * GD Version : bundled (2.0.34 compatible)
    * FreeType Support : Yes
    * FreeType Linkage : with freetype
    * T1Lib Support : No
    * GIF Read Support : Yes
    * GIF Create Support : Yes
    * JPG Support : Yes
    * PNG Support : Yes
    * WBMP Support : Yes
    * XPM Support : No
    * XBM Support : Yes
    * JIS-mapped Japanese Font Support : No

The problem is that NextGen becomes unresponsible when I try to apply bulk action such as Change Picture Size or Apply Watermark. I just receive popup message saying something like ‘applying watermark…’, then percents increase up to 100%, but this popup never disappears, and actually no changes are being made.

Any suggestions? I appreciate your help in advance!