Transfer to an extension goes direct to voicemail

Core 14.0.28.35

FreePBX Framework 14.0.13.24

When anyone attempts to transfer a call to another extension, it goes directly to voicemail and we see the following in the log:

WARNING[16707][C-0000a98a] ast_expr2.fl: ast_yyerror(): syntax error: syntax error, unexpected '<token>', expecting $end; Input:
WARNING[16707][C-0000a98a] ast_expr2.fl: If you have questions, please refer to https://wiki.asterisk.org/wiki/display/AST/Channel+Variables

The environment is a mixture of SIP and PJSIP devices. Is there an incompatibility between those channel drivers?