A fast fix would be the modification of the file ~site/share/check_mk/web/plugins/wato/mkeventd.py around line 737 and add the size for this field:
( "match_application",
RegExpUnicode(
title = _("Match syslog application (tag)"),
help = _("Regular expression for matching the syslog tag (case insenstive)"),
mode = RegExp.infix,
case_sensitive = False,
size = 64,
)