Compatibility issue with NextGen Gallery?

I’m having some problems with the NextGen Gallery plugin. The plugin check tells me this:

Check theme compatibility
Missing the call to wp_footer() in your theme, contact the theme author

I’m not sure if this is related to my problems, but the message tells me something is wrong.

Any ideas?

This is the link of my photo page:
http://ustafeltennis.nl/?page_id=46

Selecting an album results in php errors:
Warning: preg_match() [function.preg-match]: Compilation failed: unrecognized character after (?< at offset 21 in /var/www/vhosts/ustafeltennis.nl/httpdocs/wp/wp-content/plugins/nextgen-gallery/products/photocrati_nextgen/modules/router/mixin.url_manipulation.php on line 154

Other info:
Theme: Harmony 2.0, version 1.1
NextGen Gallery: 2.0.17

Operating System : Linux (32 Bit)
Server : Apache/2.0.54 (Fedora)
Memory usage : 26.72 MByte
MYSQL Version : 5.1.47
SQL Mode : Not set
PHP Version : 5.3.10
PHP Safe Mode : Off
PHP Allow URL fopen : On
PHP Memory Limit : 48
PHP Max Upload Size : 10M
PHP Max Post Size : 8M
PCRE Backtracking Limit : 500000
PHP Max Script Execute Time : 25s
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
JPEG Support : Yes
PNG Support : Yes
WBMP Support : Yes
XPM Support : No
XBM Support : Yes
JIS-mapped Japanese Font Support : No